What if

The countries outside Marley and Eldia have joined forces with each other,to exterminate Marley and Eldia's Titans,and islands as a whole

As a result,Marley and Eldia have set apart their rivalry,and become one Nation,to stand a chance against the opposing counties

The battle took place in a desert. The enemy brought Anti-Titan Rifles. The Survey Corps,its Titans,Marley's Military and its Titans,stand,preparing for the incoming fight