The open door policy was challenged by what country with its invasion of manchuria in the 1930’s

because the vietnam war was fought in an especially brutal and untraditional way

answer: ensure protection from communism

explanation:

the north atlantic treaty was largely dormant until the korean war initiated the establishment of nato to implement it, by means of an integrated military structure: this included the formation of supreme headquarters allied powers europe in 1951, which adopted the western union's military structures and plans.