Motivator... is not a good word choice. Not what I meant to say at all. I mean it is a very profitable system that economics are involved in. I don't know why I said "Motivator." I have some very odd word choices.
You cannot trade with someone you are at war with, by the way. Also, blockades ensure that some people can't trade. World War 1, the British effectively blockaded the Germans to 0$ in trade with countries it was not at war with, and was causing mass starvation because of it. In WW1 the Americans may have WASP connections with the British, there were a significant number of German and Irish descendants who would have been far more supportive of the German side in WW1, but trade would dictate that you go to war with the British (Huge boost in trade due to being unable to meet Britian's production needs compared to total shut down of trade with Germany already present). Though the Zimmerman Tellgram did the Germans no favors.
Though do note the words "Unlikely." Declaring war on a trade partner that would cripple your economy is kind of... stupid. I didn't mean trade stopped with everyone. "The french and Germans went to war, so the Italians and Spanish can't trade" or something like that. Blockades and total war tactics can cripple trade however during wartimes with unrelated members of the war. It was important to the North to Blockade the south during the war to reduce the effectiveness of their economy and lower the reason why someone might want to help them.
Though I am stating myself pretty poorly.