Syria and Iran recently signed a defense pact.
There's a good chance that if you invade one you get the other.
That's said, Syria is the smaller and could be captured quickly. The people of Iran would welcome the US more than Syria, but because of it's size we would get more fight.
I'm certain that the each would come to the others' aide following attack by the US Armed Forces. LOL!
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Four questions:
1. Where will we get the multiple divisions needed to do this?
2. Taking Iraq was not the problem, controlling Iraq is. What would make Syria different? And, what units do we use to occupy it?
3. How do you keep Israel from being pulled in? Don't fool yourself into thinking this would be a good thing.
4. Look west and invade Syria, what do we do then when Iran opens an Eastern front?
Got any answers?