D. US troops, following orders from President Polk, claimed an area of disputed land for the United States which caused an attack by Mexico in retaliation
Explanation:
The major cause of the war was actually the annexation of Texas by the US in 1845. Among tensions, skirmishes and a failed diplomatic mission, president Polk ordered American troops commanded by Gen. Zachary Taylor to occupy the disputed zone between Río Nueces and Río Grande (Río Bravo for Mexicans) in January 1846. Taylor´s men were attacked by Mexican troops in April killing or injuring 16 of them. Polk claimed Mexican soldiers had attacked US troops and "shed American blood upon American soil." He requested a declaration of war. Congress overwhelmingly voted to declare war on Mexico on May 13, 1846.
