ANSWER:
Japan joined the Tripartite Pact, which made United States to worry because Tripartite Pact is the agreement between Germany, Italy and Japan.
Step-by-step explanation:
- United States thought that after this agreement, Japan would supply the military force and power to European countries.
- Due to this agreement the European country was having the support of Japan in field of military and this made United States to worry.
- The Tripartite Pact is also called as the "Berlin Pact".