Texas was admitted to the Union. Mexico breaks off diplomatic relations with the U.S., but, Mexico is angry because Texas was still considered to be Mexico Territory.
Tension between Mexico and America rose and they created boundaries between the two rivers, the Rio Grande and the Nueces River. Both Mexico and America had conflict once they both claimed ownership of both lands. President Polk then told General Zachary Taylor to move his 3000 troops across the Nueces River.
NO! This is our land that you're standing on!
President James K. Polk
This is our land!
Fine! General Taylor is coming with his troops to try and stop this mess!
After General Taylor's troops were set out to move from the Nueces River to the Rio Grande, a large Mexican force defeated an advance scout of U.S. cavalry troops and attacked the U.S. soldiers. President Polk then decided to declare war on Mexico by asking Congress to do so. They considered Polk's request and passed a declaration of war on May 13, 1846.
Ok. We resign. We will pass a declaration of war considering you're request.
Congress! I declare war on Mexico! They've gone too far and attacked our U.S. soldiers.