Polk had received word of the Thornton…
May 1846 CE
Polk had received word of the Thornton Affair, which, added to the Mexican government's rejection of Slidell, Polk believes, constituted a casus belli (cause for war).
He had presented his war message, two days after receiving news of the initial Mexican attack, to Congress on May 11, 1846, claiming that "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil."