WASHINGTON — In a grim warning about financial priorities during a time of fiscal crisis, the defense secretary, Robert M. Gates, said Sunday that budget pressures must not limit Pentagon spending such that the military is unable to defend American interests in an unpredictable world.

Most notable for Mr. Gates — a longtime advocate for diplomacy and development, along with military power, to protect American global interests — was his reminder of the long-term requirement for the United States to sustain an armed force superior to any adversary.

“The problems we as a nation are grappling with are well-known: steep fiscal imbalances and mounting debt, which could develop into a deep crisis for our country,” Mr. Gates told graduating seniors at the University of Notre Dame.

He went on: “At the same time, we face a complex and unpredictable international security environment that includes a major war in Afghanistan, winding up the war in Iraq, revolution throughout the Middle East, new rising powers, nuclear proliferation in Iran and Korea, the continued threat of terrorism, and more.”