When a country declares a another country, they are refusing to trade with them

I'm pretty sure the right answer that you're looking for is embargo.

embargo is when a country shuts down their trading system to another country.

albany plan of union, 1754. the albany plan of union was a plan to place the british north american colonies under a more centralized government

when world war i broke out across europe in 1914, president woodrow wilson proclaimed the united states would remain neutral, and many americans supported this policy of nonintervention. however, public opinion about neutrality started to change after the sinking of the british ocean liner lusitania by a german u-boat in 1915; almost 2,000 people perished, including 128 americans. along with news of the zimmerman telegram threatening an alliance between germany and mexico, wilson asked congress for a declaration of war against germany. the u.s. officially entered the conflict on april 6, 1917.