World War II was the last constitutionally declared war of the 20th century.
World War II was the last (and perhaps only) war of the 20th century in which American Christians who held to “biblical just war theory” were largely in agreement among themselves as to the validity of the war. Though legitimate issues were under consideration prior to the Japanese attack, the debate over whether such a conflict was “biblical” ended on December 7, 1941.
World War II was the last war in which the American defenders were born from parents who were themselves born in the pre-modern world.
World War II was the last completed war in which the United States had specific military objectives for total victory, and won a clear and decisive military and political victory over the enemy. (This was not true of Korea or Viet Nam. The Gulf War failed to topple the enemy government. The Iraqi conflict is still in motion.)