What action caused Polk to decide to ask for a declaration of war?

Answer 1

I presume you're asking about President James K. Polk and the Mexican American War ?

After the Texas annexation, Polk turned his attention to California, hoping to acquire the territory from Mexico before any European nation did so. In 1845, he sent diplomat John Slidell to Mexico to purchase California and New Mexico for $20-30 million US dollars. Slidell's arrival caused political turmoil in Mexico after word leaked out that he was there to purchase additional territory and not to offer compensation for the loss of Texas. The Mexicans refused to receive Slidell, citing a technical problem with his credentials. In January 1846 to increase pressure on Mexico to negotiate, Polk sent troops under General Zachary Taylor into the area between the Nueces River and the Rio Grande—territory that was claimed by both the U.S. and Mexico.

Slidell returned to Washington in May 1846, having been rebuffed by the Mexican government. Polk regarded this treatment of his diplomat as an insult and an "ample cause of war", and he prepared to ask Congress for a declaration of war. Coincidentally, mere days before Polk intended to make his request to Congress, he received word that Mexican forces had crossed the Rio Grande area and killed eleven American troops. Polk then made this the casus belli, and in a message to Congress on May 11, 1846, he stated that Mexico had "invaded our territory and shed American blood upon the American soil." He did not point out that the territory in question was disputed and did not unequivocally belong to the United States. Several congressmen, including a young Abraham Lincoln, expressed doubts about Polk's version of events, but Congress overwhelmingly approved the declaration of war,