Where can you get good scenarios for science fiction or modern Cold War wargames? How about the news? Reports of missiles being fired at aircraft. Reports of space ships landing. Reports of space ships crashing.
Change the word alien and substitute the word Spetznaz. The Japanese in WWII had aircraft that would fold up and fit into a pressurized cabin on the deck of a submarine. They would surface the sub and assemble the plane and fly missions to bomb places or scout for enemy fleets. How about a Soviet sub with a similar cabin, they assemble a helicopter, and send troops in ahead of a major attack. Maybe days or weeks before a real attack.
You tell your NATO player that he gets a call of a UFO and space aliens. Maybe the Spetznaz are wearing MOPP gear so they appear alien. Farmers can shoot at them, the police can respond, gunfire between the police and Spetznaz. It could take a long time for the military to respond. Russian player has to take out the nuclear power plant, or the missile defense system, or big radar array. After that they have to go to the airport and steal a helicopter since theirs crashed.
Scenarios are not too hard to put together, you just have to look at the news and then reconfigure the events to fit you time period and the troops you want to use. Instant scenario.
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