They floated down from the sky Sunday — 2,000 mice, wafting on tiny cardboard parachutes over Andersen Air Force Base in the U.S. territory of Guam.

But the rodent commandos didn't know they were on a mission: to help eradicate the brown tree snake, an invasive species that has caused millions of dollars in wildlife and commercial losses since it arrived a few decades ago.

That's because they were dead. And pumped full of painkillers.

The unlikely invasion was the fourth and biggest rodent air assault so far, part of an $8 million U.S. program approved in February to eradicate the snakes and save the exotic native birds that are their snack food.