Too late to change to Syria?

Entry into an existing conflict is a response informed by the presence of an interstate conflict, and is therefore a response to it.
My understanding of of the War in Afghanistan (however limited it might be) is that the US's entry was, supposedly, a response to the extant Afghan civil war, which would make it a response to
intrastate conflict, or, if you prefer, a response to the global threat of terrorism. The participation of countries in support of the US following this first invasion would then be responses to interstate conflict, as they were informed by the situation.
The only reason the US would be excluded from being a response to an interstate conflict is the fact that there was no interstate conflict between Afghanistan and some other state at the time they invaded. Not because they were participants.