Why did the United States and the Soviet Union have to proceed more carefully in international affairs after both developed nuclear weapons?

Because they didn't want the other to get spooked and send a nuke towards the other in response.

The world has changed a great deal in the last decade and a half. The Cold War stand-off

with the Soviet Union is over, and Russia is no longer an ideological adversary. The

United States has made historic reductions in its operationally deployed strategic nuclear

forces and plans to reduce them to a level of 1,700 to 2,200 by 2012, as called for by the

Moscow Treaty. The U. S. has also greatly reduced its non-strategic nuclear forces and

the total nuclear warhead stockpile. These significant nuclear reductions are fully

warranted in the new security environment.

The United States continues to maintain nuclear forces for two fundamental reasons.

First, the international security environment remains dangerous and unpredictable, and

has grown more complicated since the dissolution of the Soviet Union. Political

intentions can change overnight and technical surprises can be expected. Second, nuclear

weapons continue to play unique roles in supporting U. S. national security. Although not

suited for every 21st century challenge, nuclear weapons remain an essential element in

modern strategy.