“If (Syria has) WMD (especially leftovers from Iraq) then it’s our business.”
Only if they use them. Merely having chemical or biological weapons in that particular neighborhood is just a good idea given how often they’ve been used around there.
Keep in mind that Syria is next door to Turkey which sees Syria as something that they want to take back some day.
On the contrary, I think the US needs to get involved before WMD are used, not after.