We usually think of bridges are source of connection—and indeed, they are—so when Ellie and the gang blow up the bridge the soldiers are using to truck in ammo and military supplies, well, we know they've thrown a major wrench in the invaders' plans. Unable to easily re-up their supplies now, a massive group of soldiers is pretty easily outsmarted by a group of gangly teenagers. Ha.
This bridge isn't just here to represent rupture, though: In disconnecting the soldiers from their supplies, the teens take a step toward reconnecting themselves with their families and their community with their freedom. Is everything suddenly all good? Nope, but a step has definitely been taken, and now the invaders know they aren't going to just keep on keeping on without being challenged: There are still people on the loose who are so connected to their lives here that they're willing to throw down big time to defend them.