Of our 13 ground combat divisions (active duty), there are of course scenarios where most would deploy. The worst would be a Soviet invasion of say Lithuania or Estonia or Latvia, or perhaps Poland. I don't view that as likely, the Russkis are not in good shape.
Another would be the Korean peninsula, I don't view that as likely either. Another would be Taiwan, though we'd probably try and maintain distance there, not ground forces.
One reason these things are not likely is those 13 divisions.
We went into Iraq with 2 divisions reinforced as I recall, 4th ID was held up in Turkey. That was plenty to destroy the Iraqi military and not nearly enough to stabilize the country. If you break it, you own it.
I lean to think we could go to a larger reserve force and smaller active, but I'm not sure about that of course. The IRR is there as well, and then the NG, which is not in great shape according to one member of my family.
Not sure which Iraq war you are citing, CD.
In 1991, the good guys' order of battle included 8 U.S. Army divisions, 3 U.S.M.C. divisions, a French division, a UK division, plus 2 U.S. armored cavalry regiments (about 2/3 of a division total), and about 5 1/2 Arab division equivalents. Plus a huge number of troops in corps artillery and aviation brigades, not to mention intel, engineer, transportation, supply, maintenance, medical, etc. units. Or the USAF and USN.
In 2003, the good guys' order of battle included 4 U.S. Army divisions, a reinforced U.S.M.C. division plus a MEB, a UK division, plus about a division equivalent in the Special Ops Command. Plus all the assets at Corps level and higher to support the front-line guys. The 4th Div was late, awaiting all its vehicles, which our friendly NATO ally Turkey would not allow to transit, but it did get into the race to Baghdad. All in all, it was about half of what we put into Iraq in 1991, and that's not counting the lack of allied Arab units that we had had in the earlier campaign.
You probably recall that there was much controversy over how much weaker our force was in '03 compared to '91.