These groups include Shi’a militias from Lebanon, Syria, and Iraq, as well as some Iranian regulars. The reality is that Iran, not despite the Nuke deal, but in many ways because of it, is cementing its control from Iran to the Mediterranean. When Hezbollah forces return to Lebanon, they will dominate it. The same will be true in Syria and Iraq.
Where will the Iranians focus their attention when Assad ultimately regains control of Syria? They are already stirring up trouble in Yemen. Their next targets could be Bahrain, Saudi Arabia, Jordan, or Israel. When the Syrian civil war concludes, Iranian controlled forces will be arrayed along Israel’s northern border, and in Gaza.
What should the US do? There are no easy solutions, but any policy that does not include possible military action against Iran will not succeed. Obama already tried the appeasement option with his Nuke deal. That didn’t work. So the only remaining option is to isolate and counter Iran, directly or indirectly. That’s not a simple task.