I would speculate that this question is confusing to you because it seems to imply that President Polk was the sole cause of the Civil War or that his actions led directly to the Civil War. This is, of course, not true. However, Polk’s actions did help very much to bring the war about. Specifically, Polk’s annexation of Texas and the subsequent war with Mexico helped to cause the Civil War to happen.
In the 1830s, Texas had won a war with Mexico that had left Texas as an independent country. Texas was populated mostly by American settlers who had been invited in by the Mexican government. The independent Texas government asked to become part of the United States but was rejected for about 10 years. In 1845, Polk annexed Texas. This led to a war with Mexico that began in 1846 and ended in 1848. The US won the war and forced Mexico to accept the annexation of Texas and to give the US huge areas of land in what is now the Southwestern US.
This war helped cause the Civil War because of the issue of slavery in the areas taken from Mexico. Many people in the North felt that Polk (a slaveowner from the South) wanted the war as a way to add more territory that could have slaves. In other words, they felt that the whole point of the war was to help spread slavery and give the slave states more power. This made them very angry. Because of this, the issue of slavery in the Mexican Cession became a very incendiary one. The conflict over the issue pushed the North and South farther apart, helping to lead to the Civil War.
Polk’s War (as some in the North called it) also led to other events (after Polk’s presidency) that helped bring the Civil War about. First, it led to the Compromise of 1850. This compromise had to do with the issue of slavery in the Mexican Cession. It helped cause the Civil War largely because of conflict over the Fugitive Slave Act, which was a part of the Compromise. Second, gold was found in California (part of what was taken from Mexico) in 1849. This led to the Gold Rush and helped cause more of a need for a transcontinental railroad. The need for the railroad helped to bring about the Kansas-Nebraska Act of 1854. This act destroyed the Missouri Compromise and helped to make the North and the South still angrier at one another.
Thus, Polk’s actions caused the North and South to come into greater conflict with one another. Polk’s actions did not cause the Civil War on their own, but they did help very much to create animosity between the two sections and that animosity eventually led to war.