IN HIS SPEECH to the British Parliament on Wednesday, President Obama rejected the notion that American and European influence must decline amid fiscal crisis and the rise of powers such as India and China. “The time for our leadership is now,” he declared. “Our alliance will remain indispensable to the goal of a century that is more peaceful, more prosperous and more just.” We agree with him — which is why we hope Mr. Obama will carefully weigh the farewell policy address delivered one day earlier in Washington by his defense secretary, Robert M. Gates.

Mr. Gates, too, believes that “America does have a special position and set of responsibilities on this planet.” But his speech to the American Enterprise Institute focused on the connection between that continuing global responsibility and the coming cuts to the U.S.. defense budget. Mr. Obama has proposed that $400 billion be sliced from national security spending over the next dozen years, with most of it coming from the Pentagon.

Mr. Gates, who has spent the past year trying to rationalize defense spending, acknowledges that that cutback would not be drastic; he reckons that it would mean holding defense spending just below the rate of inflation. But he is blunt about what it would mean: “A smaller military,” he said, “will be able to go fewer places and do fewer things.”

The defense secretary cited a couple of reasons that even moderate budget reductions could have a big impact. One is the rapidly inflating costs of weapons systems such as the new F-35 warplane: “More and more money is consumed by fewer and fewer platforms that take longer and longer to build,” he said. Yet modernization is critical: The military needs not just the F-35 but a new fleet of refueling tankers and a new generation of submarines to retain its edge over China and Russia.

Another big problem is the Pentagon bureaucracy, which has resisted Mr. Gates’s attempts at cuts. He said he managed to cut only $1 billion from the $64 billion spent on the “fourth estate” of Defense Department offices outside the military services. To preserve the funding needed for weapons modernization and avoid cutting too deeply into the forces, Mr. Gates’s successor will have to address what he described as “too many headquarters, offices and agencies employing too many high-ranking personnel and contractors.”

Congress, too, will have to change its usual priorities if defense cutting is to be compatible with continued U.S. strength. Not for the first time, Mr. Gates pointed out that spending on military health care is unsustainable; that the retirement and pension system is riddled with perverse incentives; and that military salaries should be looked at in light of the fact that all of the armed services have consistently exceeded their recruiting and retention goals at current pay levels.

Even if all of the Pentagon’s inefficiencies and sacred cows are tackled, the defense cuts proposed by Mr. Obama pose a fundamental dilemma. Said Mr. Gates: “The tough choices ahead are really about the kind of role the American people — accustomed to unquestioned military dominance for the last two decades — want their country to play in the world.” Mr. Obama outlined an ambitious vision of that role in London. If he means it, he will have to work with Mr. Gates’s successor to ensure that the Pentagon is able to retain enough forces and build enough planes and ships to make it possible.