Final answer:
President Polk ordered General Zachary Taylor and US soldiers into Texas to provoke war with Mexico, taking advantage of the disputed territory between the rivers. Polk declared war after a skirmish broke out between Taylor and a Mexican general, leading to Congress approving the declaration. This started the Mexican-American War.
Step-by-step explanation:
President Polk ordered General Zachary Taylor and thousands of US soldiers into Texas as part of a strategy to provoke war with Mexico. The disputed territory between the rivers was a point of contention between Mexico and the United States, with Mexico claiming that the border was the Nueces River, while the United States claimed it was the Rio Grande. Polk saw Taylor's presence as an opportunity to provoke Mexico and declared war when a skirmish broke out. This led to Congress approving the declaration of war and the subsequent military campaigns by US forces.