The Orange Box

When you have FINALLY reached the white forest base after a while you will be told to go and check the silo doors on silo 2. THe combine attack around half way through this excursion an you now have to rush to sucure the silo doors and prevent any more combine from flooding in. Once you have gotten to the top of missile silo 2 and activated the silo doors you'll get the achievement. This is story related, so if you just play through episode 2 you'll get it.