President Barack Obama has called for a one-third reduction of the world's nuclear stockpiles.
Speaking at Berlin's historic Brandenburg Gate, a powerful symbol of the Cold War, he said it was possible to ensure American security and a strong deterrent and also reduce nuclear weapons.
He said he intends to seek negotiated cuts with Russia to move beyond a Cold War nuclear posture.
Mr Obama promised to work with Nato allies to forge a new international framework for peaceful nuclear power. He said the US will hold a summit in 2016 on securing nuclear materials around the world.
He also called for a treaty to end production of fissile material.