No, sorry. Any enemies like the Enclave or the U-Foes just show up randomly in sandbox mode. Not knowing where exactly you are in the game, I will go ahead and point out that eventually you will reach a point where there is a Threat Level indicator and the Army will show up when you do enough damage (starts at Level 1)...and will get steadily worse the further into the game you get, up to a Threat Level 9 when they will send a soldier in Hulkbuster armor after you, along with every other Strike Team you've fought up to that point.....and the Enclave may show up during all that too
![Sad [:(]](/emoticons/emotion-6.gif)
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The only way to go back to the pre-Enclave free play is to completely start a new game and stay away from the green square that starts the first mission after the Brazil level at the very beginning.....
Brains are like dynamite. Some people don't have enough to blow their nose.