Answer:
The US had significantly reduced it's air force as part of it's disarmament program after WWI. Along with it's Air Force, it also slashed it's army and naval forces.
The RAF regiment remained after the war as a regular arm of the service, tasked with securing airfields and providing forward air control personnel to British army and Royal Marine ground forces.
Most French planes did remain in service after WWI. However, orders for new planes had significantly dropped eventually making the French air regiments filled with outdated planes; unfit for the second world war.
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