Why did Papa Joe and his Italians want Rocco killed by sending him to assassinate the Russians? Considering that Rocco was known to be part of their gang, did they want to start a war?
They sent in Rocco as a way of killing two birds with one stone as they most likely see him (who they have given the mocking title of the funny man) as a liability and an embarrassment. They also sent him as a form of messenger with a message of hostility as it is explained that the Russians came to Boston as it is a fertile ground for crime. And in the spirit of glasnost they tell the Italians that they may preform criminal acts in Russia with no negative repercussions but the Italians refuse this deal and the Russians continue to push into Boston which is why Rocco was sent to kill the Russians. Two birds with one stone.
I don't think this would have started a war. Rocco was known as the Funny man he was basically looked at as a joke by almost everyone. Sending the least threatening guy to shoot the Russians was basically a joke on him. On the one hand he may kill the guy he was sent for if he gets killed they can say "sorry god that idiot thought he could get Made by doing this we are so sorry he was such an idiot." They don't lose anything since they don't value or care about him. Also it lets them know how well guarded the Russians are.