Warsaw Pact Formed
1955
A reaction to perceived Western aggression by the creation of NATO, the Warsaw Pact was a Russian-dominated military alliance that included all of the states of Eastern Europe except Yugoslavia. It integrated both tactics and equipment throughout the alliance along Soviet models. Albania withdrew from the Pact in 1968.
Time: Early War
Side: USSR
Ops: 3
Removed after event: Yes
As USSR
Being able to remove all US influence in Eastern Europe is vastly superior to adding 5 influence. It is essentially Truman Doctrine at any time (regardless of control) and in four (Eastern European) countries at once.
Accordingly, I always play Warsaw Pact Formed for Ops in the Early War / Mid War so that I can keep it in the deck. Its mere presence in the draw deck is a threat to the US, in much the same way that Truman Doctrine deters the USSR from engaging in European Ops wars. Warsaw Pact is your best (and sometimes only) defense against the Late War US onslaught of Chernobyl, Tear Down This Wall, East European Unrest, John Paul II Elected Pope, and Solidarity.
Once you get to Turn 7 and draw Warsaw Pact, it is then a common USSR hold card in case of European emergency. Since any card played or discarded on Turn 7 or later is likely not going to return to the game, you must try to keep it in the game for as long as possible. As long the US player is convinced that the Warsaw Pact may yet be formed, he will be very hesitant about investing into Europe, thereby nullifying one of the great US advantages in the game.
In the unfortunate event that the US triggers Warsaw Pact Formed in the Early War or Mid War and there is no meaningful US influence to remove, I ordinarily bump East Germany and Poland by 2 each and stick the other influence in Yugoslavia or Czechoslovakia. (Yugoslavia because I usually start with 1 there, and Czechoslovakia because it could theoretically provide a -1 against a Tear Down This Wall realignment of East Germany or Poland.)
As US
If you draw this in the Early War / Mid War, count your blessings. As US, Warsaw Pact Formed is actually a better draw than most US events (like, say Five Year Plan). Being able to get rid of a critical USSR Late War card is a huge bonus, and by playing it when you have no influence to be removed, you can save yourself a lot of worry and headaches in the Late War.
As an added bonus, an early play of Warsaw Pact Formed means the USSR can no longer dump NATO or Independent Reds with impunity.
Twilight Strategy 
If USSR draws Warsaw Pact on turn 1, would it be worthwhile to make it his/her hold card twice and use it for the ops on turn 3 so that it can be guaranteed to survive to the turn 7 reshuffle? Especially since there really aren’t any horrible US events which need to be held like decolonization or de-stalinization.
Depends – Marshall Plan and Defectors are both worth holding until turn 3 if you can do it worry-free, IMO. Marshall is strong, and Defectors being in your hand on turn 3 means no worries for the whole Mid War. But it’s not crazy to think of Warsaw as being another one worth saving until the reshuffle.
I would say no. While it can have a massive impact on europe, this is only important if the US launches a full-scale attack on europe. Otherwise, the 5 IP you get if US plays it is fine. Also, it’s 3 ops, why hold it for a very hypothetical Europe defense 6-9 turns later?
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seiko laurel
So just say the US play this card, is it up to the USSR or the US to determine which part of the card will take place?
USSR plays the event, so USSR chooses.
To clarify – effects of red-starred events are always executed by the USSR player, regardless of the player who actually played the card.
Likewise for white-starred events and the US player.