Which of these events influenced the formulation of the Truman Doctrine?

Truman's background as a farmer

the dropping of the bombs on Hiroshima and Nagasaki

Truman's time as a judge in Missouri

the Soviet's plan for expansion,

The event that influenced the formulation of the Truman Doctrine is the Soviet's plan for expansion. When the Soviet planned to expand its sovereignty across the globe, then there was a formulation of the Truman Doctrine. Its purpose is to oppose to Soviet's geopolitical expansion.