I've done that, but when I deploy men to say...Turkey, they don't enter Iraq.
For example, I am in the war room, ready to send troops to Iraq. I adjust troop levels, press do it, and Turkey offers to take my troops. My troops then go to Turkey, but they don't enter Iraq, making it near impossible for me to win the war, as for some reason Iraq can still kill thousands of my men without even being able to see them.
Build up your military prescense in the region before declaring war. Iraq will ALWAYS at some point force a war, so it's best to deploy your troops to the surrounding areas before you declare war. Your military prescense must be larger than Iraq's in order to win.