The result of the Union winning the Civil War was that it was settled once and for all time that the US would be a single entity, with the Federal Government taking precedence over State and local loyalties. Prior to 1865 there were two competing strains of political thought in America which can be boiled down to a) pro-Union; and b) State-centrist. The first US government was formed under the Articles of Confederation, which was a pretty extreme form of the State-centric model. This government simply did not work, and disparate elements of the political leadership came together to write the US Constitution as a fairly desperate attempt to keep the country from spinning apart. The US Constitution was only narrowly ratified by the States because of distrust of a strong Federal government, and it was finally passed only under the condition that certain amendments would be added (later called the Bill of Rights) to mollify fears that the Federal Government would over-reach and establish a tyrannical government over the States. One of the key additional amendments that was committed to was what became the 10th amendment, which says that the Federal government will only have the powers that are specifically assigned to it in the Constitution (and therefore all other powers will reside with the States). The scope and extent to what exact powers the Federal government has over the States is still being debated to this day.

Between 1781 and 1860 there was a constant political push-pull between the Unionists and the States Rights factions. During the War of 1812 there was a serious threat by the New England states to secede due to the unpopularity of that war. Although they were political enemies, Andrew Jackson (on one hand) and Henry Clay, John Quincy Adams, and Daniel Webster (on the other hand) shared the common goal of maintaining the Union above all other political considerations. John C. Calhoun, who early in his political career in Congress and the cabinet of Monroe was a Unionist, became the staunchest States Rights politician in the country. His state of South Carolina was threatened to be militarily invaded by President Jackson due to it's enactment of laws of 'nullification' of certain Federal laws. South Carolina was the epitome of the State where the local politics were seen as taking precedence over the National political identity. Other Southern states to a greater or lesser extent followed South Carolina's lead in the States Rights stance. And South Carolina was the first state to attack the US military (at Ft. Sumter), launching the Civil War.

The final, total victory of Lincoln's Federal Government in 1865 settled once and for all whether the US would be dominated by a strong central government or whether the States would dominate the government. In the North, the victory which was fought for at such a high cost certainly cemented the idea of the Union First. In the South, there were no cohesive State governmental structures left to effectively counterbalance the power and identity of the Federal government.