Why allies still plan to fight each other

When did War Plan Red and the British equivalent stop being serious plans?

No specific end date is given. Such plans could remain valuable exercises even as Anglo-American naval cooperation deepened.

No single date is given for War Plan Red or the British equivalent ceasing to be serious planning documents. An unlikely war could remain useful as a demanding staff exercise. A strong hypothetical opponent tests logistics, doctrine, and command under pressures that routine scenarios might avoid. Anglo-American naval cooperation was already extensive by the later 1930s, including developments placed in 1936. The continuing usefulness of a scenario is therefore separate from a government's expectation that the war will occur.