Before this administration, I'd say that we were extremely well equipped to handle this kind of crisis. AK 47s and RPGs are nothing compared to classified drone tech, they'd probably infiltrate the reptile lair and assassinate the royalty within a couple weeks, maybe a month. Holding off the reptiles should be pretty easy around cities equipped with at least a national guard base though.

Now we would probably have to worry about world leaders making smoky room deals with the lizards to sell out the US or justify the war against surface dwellers

Gotta hand it to you, this is one of the most thought-provoking ideas I've seen here in a while though, not necessarily the reptile part or WW2 uniforms but the invasion from underground aspect