Actually, the Republic came to an end in November of 1860. Forget the pablum you learned about the American Civil War in school and see if some of this version might sound more plausible.
Before the Lincoln Administration: the USA was far less "united" than it is now. Technologies like railroads, mass production, and the telegraph were causing tremendous changes in the Northern states, less in the South. The country was still legally structured for wagon transport of goods, farming, and couriers on horseback. A corporation wishing to buy farm goods in Indiana and then ship them to Philadelphia faced many hurdles. Favorable prices in Philly could be telegraphed quickly to buyers in Indiana, the goods could be shipped to Philly within two days and a major profit could be reaped. Unfortunately for the corporate types, the banks of the time each issued their own paper currency that had the full backing of that particular bank. To cope with this, there were a few banks that were legally allowed to accept out-of-state bank currencies and allocate them a value based on the perceived ability of that bank to meet its' obligations. In other words, the "American Dollar"essentially had to be excahanged like a foreign currency just to buy and move goods 50 miles quite often. It would be like having to wire transfer cash to your seller after buying something on the Internet. The US Dollar was not the only currency in use, either. British pounds and Spanish pesos were routinely used for buying and selling goods, especially in the South which exported tremendous amounts of cotton to Europe. There were no Federal banks and there was no Federal Reserve bank. Andrew Jackson had destroyed the original Federal Reserve bank which had been started by early Big Business interests that had backed his predecessor. Jackson had been the advocate for the expansionist, Western and farming interests. Additionally, to man all those factories up North, immigration policy was set to wide open; Northern Europeans poured into the major Northern port cities. They were changing the character of the country to a more industrial, multicultural one.
The agricultural, tradition-minded South and its' horrible, legally-sanctioned practice of exchanging human chattel stood in sharp contrast to the rapidly-evolving Northern states. Indentured servitude was still legal in the North, and working conditions in many of the Northern factiories were appalling. At least the Northern laborers could not be legally whipped to death. The rapid growth of the American economy called for vast pools of low-cost labor and both North and South had found ways of getting it. Fortunately, the South's pool of labor was static as the English had outlawed the slave trade around 1830 and the Royal Navy was able to enforce that against the Boston slave merchants who carried the slaves from West Africa. It was the children of these Boston slave merchants who had grown up in houses built on the profits from slave trading who spearheaded the abolitionist movement. These Abolitionists eventually became the moral force of the Republican Party which had picked up the corporate and Unionist support left high and dry by the dissolution of the Whig Party. The Republicans united the worthy goal of ending slavery with the crass goal of making America safe for the McDonalds, the Union Pacific Railroad and the Fedex's of the future. And so the stage was set for the Election of 1860 as a political party bent on turning the US into a single trading zone seized the reins of power it won that November. Five years later, we had a Federal Reserve banking system, a national trade school system (the Agricultural and Mechanical colleges mandated under the Morrill Act of 1962), and over a million people had died. The war itself had been a huge boon for nationalization and corporate profits. The corruption of much of the Lincoln Administration was so bad that soldiers would find in the heat of combat that their blackpowder had been filled out with dirt. At least slavery had been abolished, but the freed slaves were left high and dry to rot under the apartheid system of "separate but equal" which was given the Supreme Court seal of approval in Plessy v. Ferguson of 1894. This may sound strange for America, but the corporations got their way during and after the war while the freed men had to wait until the 1960's for real freedom.
Once the consolidation of power was completed in the late 1800's, America moved on to Empire under the initial leadership of Teddy Roosevelt. Under George W. Bush, the Empire seems to have reached its' zenith. Many historians believe America would be anything but "the world's policeman" if the Civil War/War Between the States had not happened. The American Empire is centered on trade, much like the British Empire, but is less oppressive overseas.
An irony of the current "atmos-fear" is that the danger of world annihilation has faded with the end of the Cold War, but thanks to the occasional terrorist mass murder incident, there are still dangers for this empire. When the Cold War ended, experts were polled to rank the world's intelligence agencies. The finish was: KGB #1, Mossad #2, MI-6 #3 and CIA #4. Al Qaeda is a serious threat to this Empire, especially as its many enemies may coalesce into alliances. The difference from earlier rivalries is all the trade links between nations that might otherwise be obvious foes. Maybe the victory of the corporation (a.k.a. "globalization") can prevent widespread future wars.