I honestly think this war would be tough, Earth has better war technology however Mewni has magic. I do wonder who would prevail, although i'd have to go with Earth, because they can take over the sky without opposition, because they have jets and helicopters, and Earth also has a navy, that Mewni does not. Mewni still fights with old tactics in Phalanx formations, a battalion of M1 Abrams could plow right through a legion of Mewman soldiers, cavalry wouldn't be able to put up a fight due to the fact that Earth has tanks and armored vehicles. Mewni would quite honestly get steam rolled by Earths military technological advantage, that's even with magic, since only the queen and her heir have the ability to use it, a fight would be heavily dependent on magic to win. Not trying to bum it out, just being realistic, now if Earth were still in it's dark ages then that'd be a different story, they'd mop the floor with us. Then again we wouldn't have a way of opening a portal to Mewni unless we master magic as well and by that point Mewni would be screwed.