Soli Deo Gloria!
There are some events that will only happen if other cards have been played.
As an example, if your playing Russia and you have the Marshall Plan and Nato cards in your hand you can play them without triggering the event and they go to the discard. It will pass some turma until they papear again.
No, Marshall Plan doesn't have a prerequisite. If you play NATO first, then it will not trigger, since it depends on Marshall Plan. However, Marshall Plan will always occur if played for ops (barring UN Intervention).