The word democracy is misused, but putting that aside and rephrasing the question so as to convey the modern misused meaning of democracy, I can then attempt to offer a competitive answer.

Original question:
"Was the US Civil War the death knell of the Republic and the rise of the Democracy"

Rephrased question:
Was the US Civil War the death knell of the Republic and the rise of a form of tyranny known as Majority Rules?

No.

The Republics ended in 1789 when the criminal Federalist Party Unified all the Republics into one Profitable Monopoly Nation State, which then ensured a Civil War.

That is the factual answer, it may not be what people want to hear.

A Republic is The Public Thing, not the thing used by the Elite to enforce their tyrannical forms of slavery upon everyone.

That was once known:

Thomas Paine Rights of Man
Chapter III
Page 176

"As it is necessary to clear away the rubbish of errors, into which the subject of government has been thrown, I will proceed to remark on some others.

"It has always been the political craft of courtiers and courtgovernments, to abuse something which they called republicanism; but what republicanism was, or is, they never attempt to explain. let us examine a little into this case.

"The only forms of government are the democratical, the aristocratical, the monarchical, and what is now called the representative.

"What is called a republic is not any particular form of government. It is wholly characteristical of the purport, matter or object for which government ought to be instituted, and on which it is to be employed, Res-Publica, the public affairs, or the public good; or, literally translated, the public thing. It is a word of a good original, referring to what ought to be the character and business of government; and in this sense it is naturally opposed to the word monarchy, which has a base original signification. It means arbitrary power in an individual person; in the exercise of which, himself, and not the res-publica, is the object.

"Every government that does not act on the principle of a Republic, or in other words, that does not make the res-publica its whole and sole object, is not a good government. Republican government is no other than government established and conducted for the interest of the public, as well individually as collectively. It is not necessarily connected with any particular form, but it most naturally associates with the representative form, as being best calculated to secure the end for which a nation is at the expense of supporting it.

"Various forms of government have affected to style themselves a republic. Poland calls itself a republic, which is an hereditary aristocracy, with what is called an elective monarchy. Holland calls itself a republic, which is chiefly aristocratical, with an hereditary stadtholdership. But the government of America, which is wholly on the system of representation, is the only real Republic, in character and in practice, that now exists. Its government has no other object than the public business of the nation, and therefore it is properly a republic; and the Americans have taken care that this, and no other, shall always be the object of their government, by their rejecting everything hereditary, and establishing governments on the system of representation only. Those who have said that a republic is not a form of government calculated for countries of great extent, mistook, in the first place, the business of a government, for a form of government; for the res-publica equally appertains to every extent of territory and population. And, in the second place, if they meant anything with respect to form, it was the simple democratical form, such as was the mode of government in the ancient democracies, in which there was no representation. The case, therefore, is not, that a republic cannot be extensive, but that it cannot be extensive on the simple democratical form; and the question naturally presents itself, What is the best form of government for conducting the Res-Publica, or the Public Business of a nation, after it becomes too extensive and populous for the simple democratical form? It cannot be monarchy, because monarchy is subject to an objection of the same amount to which the simple democratical form was subject.”

If a question is asked one can assume that the accurate answer is desired.

Note in the above information offered by someone who clearly knows what he is writing about, not some troll on the internet, the following:

"Every government that does not act on the principle of a Republic, or in other words, that does not make the res-publica its whole and sole object, is not a good government. Republican government is no other than government established and conducted for the interest of the public, as well individually as collectively. It is not necessarily connected with any particular form, but it most naturally associates with the representative form, as being best calculated to secure the end for which a nation is at the expense of supporting it."

When the criminals took over in 1789, which then ensured a Civil War, they made African Slavery a National Subsidized Privileged Corporate Business: a national government enforced monopoly. It was not a Republic from then on, it was of, for, and by criminals.

The point with that factual information is to point out that Republics before 1789 included the Republic of Rhode Island where former slaves could find sanctuary in an actual Republic, where The Public is actually served, including service to people who were running from slavery running from despotic non-republics.

How does one know that a Republic is not one? Subsidizing Slavery is a clue.

No. 15 – Rhode Island Is Right!

"This essay appeared in The Massachusetts Gazette, December 7, 1787, as reprinted From The Freeman's Journal; (Or, The North-American Intelligencer?)

"The abuse which has been thrown upon the state of Rhode Island seems to be greatly unmerited. Popular favor is variable, and those who are now despised and insulted may soon change situations with the present idols of the people. Rhode Island has out done even Pennsylvania in the glorious work of freeing the Negroes in this country, without which the patriotism of some states appears ridiculous. The General Assembly of the state of Rhode Island has prevented the further importation of Negroes, and have made a law by which all blacks born in that state after March, 1784, are absolutely and at once free.

"They have fully complied with the recommendations of Congress in regard to the late treaty of peace with Great Britain, and have passed an act declaring it to be the law of the land. They have never refused their quota of taxes demanded by Congress, excepting the five per cent impost, which they considered as a dangerous tax, and for which at present there is perhaps no great necessity, as the western territory, of which a part has very lately been sold at a considerable price, may soon produce an immense revenue; and, in the interim, Congress may raise in the old manner the taxes which shall be found necessary for the support of the government.

"The state of Rhode Island refused to send delegates to the Federal Convention, and the event has manifested that their refusal was a happy one as the new constitution, which the Convention has proposed to us, is an elective monarchy, which is proverbially the worst government. This new government would have been supported at a vast expense, by which our taxes - the right of which is solely vested in Congress, (a circumstance which manifests that the various states of the union will be merely corporations) - would be doubled or trebled.

"The liberty of the press is not stipulated for, and therefore may be invaded at pleasure. The supreme continental court is to have, almost in every case, "appellate jurisdiction, both as to law and fact," which signifies, if there is any meaning in words, the setting aside the trial by jury. Congress will have the power of guaranteeing to every state a right to import Negroes for twenty one years, by which some of the states, who have now declined that iniquitous traffic, may re-enter into it - for the private laws of every state are to submit to the superior jurisdiction of Congress. A standing army is to be kept on foot, by which the vicious, the sycophantick, and the time-serving will be exalted, and the brave, the patriotic, and the virtuous will be depressed.

"The writer, therefore, thinks it the part of wisdom to abide, like the state of Rhode Island, by the old articles of confederation, which, if re-examined with attention, we shall find worthy of great regard; that we should give high praise to the manly and public spirited sixteen members, who lately seceded from our house of Assembly [in Pennsylvania]; and that we should all impress with great care, this truth on our minds - That it is very easy to change a free government into an arbitrary one, but that it is very difficult to convert tyranny into freedom."

Again, the assumption is that the question asked is worthy of a competitive answer. The above points out the fact that previous to 1789 the Federation afforded the free travel of slaves from slavery to sanctuary, which was, in fact, a functioning Federation of competitive Republics, along with a few Despotic States that enforced the subsidized enslavement of some people by other people: no longer republican in nature, rather merely criminal in nature.

It is, as stated, ridiculous to claim patriotism to republicanism while making everyone pay for the enforcement of a Corporate Crime such as was African Slavery, or as it is now Corporate Debt Slavery.

So what did cause the so-called Civil War?

Again the answer is not cheap, one has to actually look at the data that proves the case:

No. 3 - New Constitution Creates A National Government; Will Not Abate Foreign Influence; Dangers Of Civil War And Despotism

Maryland Gazette and Baltimore Advertiser, March 7, 1788.

"There are but two modes by which men are connected in society, the one which operates on individuals, this always has been, and ought still to be called, national government; the other which binds States and governments together (not corporations, for there is no considerable nation on earth, despotic, monarchical, or republican, that does not contain many subordinate corporations with various constitutions) this last has heretofore been denominated a league or confederacy. The term federalists is therefore improperly applied to themselves, by the friends and supporters of the proposed constitution. This abuse of language does not help the cause; every degree of imposition serves only to irritate, but can never convince. They are national men, and their opponents, or at least a great majority of them, are federal, in the only true and strict sense of the word.

"Whether any form of national government is preferable for the Americans, to a league or confederacy, is a previous question we must first make up our minds upon. . . .

"That a national government will add to the dignity and increase the splendor of the United States abroad, can admit of no doubt: it is essentially requisite for both. That it will render government, and officers of government, more dignified at home is equally certain. That these objects are more suited to the manners, if not [the] genius and disposition of our people is, I fear, also true. That it is requisite in order to keep us at peace among ourselves, is doubtful. That it is necessary, to prevent foreigners from dividing us, or interfering in our government, I deny positively; and, after all, I have strong doubts whether all its advantages are not more specious than solid. We are vain, like other nations. We wish to make a noise in the world; and feel hurt that Europeans are not so attentive to America in peace, as they were to America in war. We are also, no doubt, desirous of cutting a figure in history. Should we not reflect, that quiet is happiness? That content and pomp are incompatible? I have either read or heard this truth, which the Americans should never forget: That the silence of historians is the surest record of the happiness of a people. The Swiss have been four hundred years the envy of mankind, and there is yet scarcely an history of their nation. What is history, but a disgusting and painful detail of the butcheries of conquerors, and the woeful calamities of the conquered? Many of us are proud, and are frequently disappointed that office confers neither respect nor difference. No man of merit can ever be disgraced by office. A rogue in office may be feared in some governments - he will be respected in none. After all, what we call respect and difference only arise from contrast of situation, as most of our ideas come by comparison and relation. Where the people are free there can be no great contrast or distinction among honest citizens in or out of office. In proportion as the people lose their freedom, every gradation of distinction, between the Governors and governed obtains, until the former become masters, and the latter become slaves. In all governments virtue will command reverence. The divine Cato knew every Roman citizen by name, and never assumed any preeminence; yet Cato found, and his memory will find, respect and reverence in the bosoms of mankind, until this world returns into that nothing, from whence Omnipotence called it.

"That the people are not at present disposed for, and are actually incapable of, governments of simplicity and equal rights, I can no longer doubt. But whose fault is it? We make them bad, by bad governments, and then abuse and despise them for being so. Our people are capable of being made anything that human nature was or is capable of, if we would only have a little patience and give them good and wholesome institutions; but I see none such and very little prospect of such. Alas! I see nothing in my fellow-citizens, that will permit my still fostering the delusion, that they are now capable of sustaining the weight of SELF-GOVERNMENT: a burden to which Greek and Roman shoulders proved unequal. The honor of supporting the dignity of the human character, seems reserved to the hardy Helvetians alone.

"If the body of the people will not govern themselves, and govern themselves well too, the consequence is unavoidable - a FEW will, and must govern them. Then it is that government becomes truly a government by force only, where men relinquish part of their natural rights to secure the rest, instead of an union of will and force, to protect all their natural rights, which ought to be the foundation of every rightful social compact.

"Whether national government will be productive of internal peace, is too uncertain to admit of decided opinion. I only hazard a conjecture when I say, that our state disputes, in a confederacy, would be disputes of levity and passion, which would subside before injury. The people being free, government having no right to them, but they to government, they would separate and divide as interest or inclination prompted - as they do at this day, and always have done, in Switzerland. In a national government, unless cautiously and fortunately administered, the disputes will be the deep-rooted differences of interest, where part of the empire must be injured by the operation of general law; and then should the sword of government be once drawn (which Heaven avert) I fear it will not be sheathed, until we have waded through that series of desolation, which France, Spain, and the other great kingdoms of the world have suffered, in order to bring so many separate States into uniformity, of government and law; in which event the legislative power can only be entrusted to one man (as it is with them) who can have no local attachments, partial interests, or private views to gratify.

"That a national government will prevent the influence or danger of foreign intrigue, or secure us from invasion, is in my judgment directly the reverse of the truth. The only foreign, or at least evil foreign influence, must be obtained through corruption. Where the government is lodged in the body of the people, as in Switzerland, they can never be corrupted; for no prince, or people, can have resources enough to corrupt the majority of a nation; and if they could, the play is not worth the candle. The facility of corruption is increased in proportion as power tends by representation or delegation, to a concentration in the hands of a few. . . .

"As to any nation attacking a number of confederated independent republics . . . it is not to be expected, more especially as the wealth of the empire is there universally diffused, and will not be collected into any one overgrown, luxurious and effeminate capital to become a lure to the enterprizing ambitious.

"That extensive empire is a misfortune to be deprecated, will not now be disputed. The balance of power has long engaged the attention of all the European world, in order to avoid the horrid evils of a general government. The same government pervading a vast extent of territory, terrifies the minds of individuals into meanness and submission. All human authority, however organized, must have confined limits, or insolence and oppression will prove the offspring of its grandeur, and the difficulty or rather impossibility of escape prevents resistance. Gibbon relates that some Roman Knights who had offended government in Rome were taken up in Asia, in a very few days after. It was the extensive territory of the Roman republic that produced a Sylla, a Marius, a Caligula, a Nero, and an Elagabalus. In small independent States contiguous to each other, the people run away and leave despotism to reek its vengeance on itself; and thus it is that moderation becomes with them, the law of self-preservation. These and such reasons founded on the eternal and immutable nature of things have long caused and will continue to cause much difference of sentiment throughout our wide extensive territories. From our divided and dispersed situation, and from the natural moderation of the American character, it has hitherto proved a warfare of argument and reason."
A FARMER

A despotic profitable monopoly nation-state is not a republic, it is a pyramid scheme run by psychopaths, sociopaths, and their growing army of sycophants. It is of, for, and by criminals at the expense of anyone who can continue to produce anything worth stealing, and an obvious consequence of such a criminal hierarchy is in-fighting. The lesser evil will take on the greater evil and the greater evil typically wins, but not always, as exemplified in the American defense which lasted from 1774 to 1789.