The big issue with the former Syrian government is that they were, to my understanding, the only nation with a written mutual defense pact with Iran. Other countries may support Iran in the event of a US invasion, but Syria was obligated to respond. Meanwhile, Iran would have been obligated to respond to an official US attack on Syria, which is why the US used Non-governmental proxies (ISIS and al Nusra) to attack them, with US troops moving in behind these terror squads to secure assets and supply lines for them. Well, up until Dec 2024, as Joe Biden was on his way out the door, when the US and Turkey did an air blitz on Syrian forces, wiping out the lion's share of them in about a week, collapsing the Assad regime.
edit: I assume that Iran didn't react to this direct attack on Syria by US forces because it was Biden at the 11th hour and they saw a possibility of peace with Trump. So far, their cowardice has paid off.