middle earth at the start of the fellowship's journey. Bombadil only allowed within the old forest. all of the armies within middle earth combined (Rohan, Gondor, Lothlorien, Southrons, Easterlings, Mordor, Rivendell, Dunlendings, saruman, mirkwood elves, the shire (all hobbits that fought in the war of the ring were regarded as surprisingly effective, so they aren't just destroyed in the first second) etc. etc.) along with various powerful creatures (balrog, any remaining dragons, Shelob, watcher in the water etc.)
airbender from the end of the war. the only issues i think middle earth faces are the earth benders (their fortifications are mostly made of rock) and possibly the fire nation airships, but i think a few of Gandalf's fireworks could sort them out (Hindenburg disaster...). ba sing se might be a problem for middle earth, but the oliphaunts might break through the wall. thoughts?