Why did Nazi Germany attack the Soviet Union (biggest trading partners)? For that matter, why did Alexander the Great advance up the Silk Road?
Erstwhile trading partners attack eachother all the time. Japan, prior to Peal Harbor was a major US trading partner. US embargos ended that. Embargos that were designed to get Japan out of China & Manchuria.
Governments colluding together is not the same thing as free trade. History shows that the more two nations engage in market commerce with each other, the less likely they are to go to war. War is often proceeded by protectionism and embargoes, where free trade brings peace. Forgive me for believing that free market capitalism is the best agent for peace and that war is just another failed big-government program. I thought people on this forum believed in small government. I used to be a hawk too, but people like Adam Smith and Milton Friedman convinced me otherwise.