I never expected the Syrian army to fall this quickly, but it didn’t look good in the first place.
Back when they were fighting back, the bull of the fighting force that didn’t come from Iran were formerly pretty active paramilitary groups that were incorporated into the Syrian army. After these fighter served full term in the army and got discharged, they joined up with other paramilitary or mercenaries groups, such as Hezbollah or Wagner group.
Now that practically every experience fighters on Assad’s side are busy elsewhere, the entire forces on Assad’s side are pretty much underequipped conscripted troops that have neither the skill, experiences, equipments, nor the morale close to what the previous generation of fighters processes. Now add the fact the rebel groups are unsubtly support by the Yank et.al., and…well, you can do the rest of the math yourself.
Depend on the agreement between the Overlord and the vassal.
An overlord can agree to join the vassal in all war, in offensive war only, in defensive war only, or none at all.
Likewise, an overlord can demand the same from the vassal, to join in all war the Overlord participate in, or only in either offensive or defensive war, or none at all.
You can see on the war demand page if declaring war on an empire would drag someone else into the war on their side or not.