Why Britain retained war plans against the United States

When was a war between Britain and the United States last considered?

British staffs kept contingency plans at least into the mid-twentieth century, but those plans were worst-case preparation rather than evidence that war was expected.

British staffs maintained plans for a possible war with the United States into the twentieth century, perhaps as late as NATO's creation. The existence of a plan is not proof that leaders seriously expected or wanted the conflict. Planning against the strongest plausible opponent is a way to test mobilisation, logistics, and force requirements under the worst case. Such work can persist after political relations make the scenario remote because staffs need demanding benchmarks. Contingency documents should not be read as direct statements of policy intent.