War plans are prudence, not proof of intended war

Do Anglo-American war plans show that a war in the 1920s or 1930s was likely?

No. Major militaries keep contingency plans for dangerous possibilities; the plans acknowledged the scale of the scenario rather than predicting it.

The existence of Anglo-American war plans did not amount to a political decision to fight. Armed forces plan for unlikely contingencies because an improbable change in another state’s behaviour could become dangerous. Britain and the United States were the two powers most capable of challenging one another, so neglecting that contingency would have been professionally irresponsible. In the interwar world, a British Empire-United States conflict would have been a dauntingly even match, which made planning sensible while war itself remained unlikely.