Which constitutional principle prohibits an individual state from declaring war?

The constitutional principle prohibits an individual state from declaring war is called federalism. The only person who can declare war is the person in charge of the government, which is the President of the United States. 

Federalism. The only person that can declare war is Congress.

The transcontinental railroad provided a means of fast transportation. it cut a 6 month trip into 6 days, created jobs for many people and encouraged immigration. the homestead act played a huge role in western expansion; it allowed settlers to begin a fresh start, and gave them land in exchange for their residence for up to five years and a small fee.