Because their racsist
1846
The Mexican War of 1846-1848 was a declared war.
On 13 May 1846, the US congress declared war on Mexico. Mexico never declared war on the United States.
because of the border problem after they got annexed
No. Texas fought Mexico in 1836 becoming its own republic that year until 1845 (Texans were actually illegal insurgents during that war as they were technically part of Mexico). In '45 Tx became part of the US. In 1846 the US (which now included Tx) fought Mexico in a legally declared war.
Germany declared war on the US, the US then declared war on Germany.
polk
The US declared war on Japan. Germany also declared war on the US, and the US declared war in return
Anglos in California declared the Bear Flag Republic immediately after The United States declared war on Mexico.
I don't think that the US declared war on Germany. Germany declared war on the US on December 11th 1941
The Italians declared war on the US on December 11, 1941, along with their German allies.
The last US declared war was WWII.