Actually if I was an opposing country, I would definatly target it out of fun and for all the confusion it might cause within the US's population. I'd reccomend surrounding them with snipers, tag the base, airstrike it (Hit those AA batteries), then maintain air superiority by shooting anything that tries to take off (The snipers can help with this too actually), then I imagine a very useful asset would be a few tanks. Problem is however, getting all this equipment so far into the US borders is going to be nearly impossible outside of wartime situations.
So realistically I'd just poison the water, the land, the air, and everyone that is seen coming in and out of the base. Move in with commandos with machine guns and gas masks at the poison's peak, take whatever there is to take, and book it. I don't know any of the surrounding towns on a personal level, so I couldn't recommend a specific place for a dropoff of the information for later retrieval, but that would be the general idea. And depending on how far emp technology has come, take out their communications that way as an added bonus.
OR send in noone less in rank than a secret agent. Disguised and unarmed. Then hope for the best.