'Blau' (i) was a German unrealised plan to check any Polish invasion of eastern Germany as part of a two/three-front war with Czechoslovakia in 'Grün' (i), France in 'Rot' (i), and Poland in 'Blau' (i) (1935/37).

The original 1935 version of the plan was based on the premise of fighting a defensive war against Czechoslovakia and Poland while defeating a surprise invasion by France.