FILTER.
Although... on the same end, their battle is not with us. While the war may get out of hand, they're not going to waste their firepower on us while they finally have the chance to claw at each other. If we bind them into the right agreement, they may be controllable. Our army could maintain the given border assuming that we only have to deal with them on one front, and assuming that they don't want to waste their time picking a fight with us, too. They would be on our turf, and that is an upper hand to us.
I can be almost certain that neither region is going to bother with using ships. I'm sure they've gone over how viable of an option it is, and if they haven't already done it they're not going to. They wouldn't be asking us for passage if they didn't deem it as the optimal choice, and if they weren't prepared to just march through us either way.