The story wasn’t really over, of course. The consequences of Texas annexation would play themselves out over the next fifteen years.
Opponents of annexation had claimed it would lead to war with Mexico, and they were right. President Polk attempted to pressure Mexico to recognize the Rio Grande as the new boundary between the United States and Mexico, and to sell California to the United States. No Mexican politician could agree to negotiate on these points and hope to remain in office. Faced with a standoff, Polk resolved to settle the matter militarily. He sent U.S. army troops under General Zachary Taylor to occupy the Rio Grande across from Matamoros. In April 1846, Mexican and U.S. troops skirmished, and Polk had his war. Major combat commenced almost immediately.
Although longer and more costly in terms of blood and treasure than the United States expected, the Mexican War ended in complete triumph for the U.S. and disaster for Mexico. In the 1848 Treaty of Guadalupe Hidalgo, the United States gained California, Arizona, New Mexico, and the Rio Grande boundary for Texas, as well as portions of Utah, Nevada, and Colorado. Because of Texas annexation, America ended up gaining a huge expansion of territory. The United States was now a true world power.
But annexation and the war had unleashed forces that no one had foreseen and no one could control. The war had been widely supported in the South and opposed in the North. Now the territory gained fell into the same seething cauldron of sectionalism. President Polk, unable to reconcile the desires of Northern abolitionists and Southern expansionists, became ill. He decided not to seek a second term and died only a few months after leaving office.
The drawn-out drama of Texas annexation had sometimes threatened to become farce. Now it evolved into tragedy. Over the next years, compromises were made and broken. Political fights turned into real mayhem and killing in Kansas and Missouri. Both sides became radicalized until political action was no longer possible.
Like the rest of the United States, Texas threw itself headlong into the maelstrom of the hateful forces unexpectedly unleashed. In the drama’s final act, Texas would join the Confederacy, and the questions raised by annexation would be settled once and for all on the battlefields of the American Civil War.