The right of a state to leave the Union was never explicitly defined in the Constitution, and thus became a highly contested issue. While some contended that the Constitution formed an unbreakable compact among the states, Southern secessionists argued that states entered the Union voluntarily and therefore retained the right to secede. However, the federal government rejected this argument, as President Abraham Lincoln declared in his First Inaugural Address that the Union could not be dissolved except by "revolution or secession."