Good question, b-c,
I think there's a sequence involved here. Everybody's been poised to go in, sort of like an invasion. The mission is on hold because of political considerations. "Send in" is sort of an idiom like "go in."
Shall we send them in now, or shall we wait awhile?
Send in the paratroopers vs. send the paratroopers. It's more dramatic with "in."
Under more normal circumstances, we'd say "Many countries are sending relief teams and disaster experts."
Prepositions lead to endless contradictory examples, but in general, I'd say you may choose to use "send in" when you're sending someone "into" some kind of a place, like a country or a house or an office.
Mr. Jones is here to see you, sir. (reply) Please send him in.
Best wishes, - A.