Allies in war

I can see two or three countries sharing the same end goal, but if there’s multiple ways to achieve that goal, how is it decided who gets the final say? In other words, how do allied countries in war decide who will ultimately call the shots? For example, Eisenhower was the commanding general of the allied forces in WW2 (if I’m not mistaken). How did he get to be so? Seems like this would have ticked off some British brass, since the Americans came into the war later.

Both the Americans and Brits knew that Americans would be providing the lion’s share of both troops and material. Churchill made a deal. Eisenhower was the Supreme Commander, but Monty commanded the troops on the ground (for a while, at least).

There are no rules that make allies stay allies. It’s like a marriage, and either party can file for divorce and walk out. The US and Great Britain could have each taken their toys and fought their own independent war. But they realized that they were FAR better off together than apart.

In the current Gulf war, there is a US marine unit (a brigade?) fighting under the command of the Royal Marines. It’s a gesture of solidarity, and sometimes of logistical necessity.

Remember that the Italians were Allies by the end of WWII, but they began it as an Axis power. Things change, the sands shift and all that.

Is there a cite for that? I know they surrendered to the Allies in 194(4?), but I never heard they re-entered the war on the other side, I could be wrong, of course.