Coordinating matters between two countries is extremely troublesome. In fact, even outside wartime, for one country's military to enter another's territory in an organized manner, numerous procedures are required.
Moreover, the war between the Dwarves and the Elves has just recently ended. Imagining what effort and coordination would be needed to let the Elf Race troops enter Dwarf territory is almost unthinkable.
This is definitely not something that General Moss could arrange with just a casual word. The Ice Cold Empire on the Dwarf side hasn't even prepared for such coordination.
Though the Elves are willing to provide all possible assistance, everything is still in the negotiation phase, as the Elves just offered cooperation within the past two days.
Even just one plane isn't ready on the Elves' side—let alone a substantial force crossing the border to fight within the Ice Cold Empire.
