Rumsfeld himself said quite openly in press conferences years ago that this was happening. I recall his comments about Saddam’s WMD’s being moved to Syria, that the US could see convoys and such by satellite, even before we went into Iraq.
So why didn’t we invade Syria too (”with us or against us” Bush Doctrine), because we didn’t have the “excuse” of “enforcing UN resolutions” like we used to put together the Coalition for Iraq?
Practically speaking, we could not “invade” Syria without Russian cooperation which will come when heck freezes over. Syria is a key Russian client and Middle East proxy. And a toolbox of useful idiots to keep Mideast turmoil going with expensive Russian weapons they buy with Iranian oil money. Does this all make sens? I didn’t think so, it doesn’t have to.
I’m sure Condi Rice the Russophile would have been advising Bush on how to stroke the Bear to keep them from aggressively interfering with us in Iraq and to try to get them to cooperate with the rational world (which the Russians have yet to enter on other than a self-interest level) on containing Iran.
Besides, Syria is small fish compared to Iran. The Israelis could neutralize Syria in about 30 minutes (maybe 15) if push comes to shove.