A serious vulnerability lurks under the surface for the Navy. It’s a problem you won’t see in New York during Fleet Week — not on the faces of the brave sailors and marines, or on their freshly painted ships, aircraft and equipment. But it is there, still, slowly eroding the Navy and Marine Corps’ ability to keep us safe from those who want to do us and our economy harm.

Since 2001, the size of the Navy has steadily been reduced from 316 ships to about 280. Yet over this period, missions and operations have increased.

In the past 18 months alone, sailors and Marines have been called upon to protect container ships from harassment and seizure by Iranian naval forces, conduct around the clock combat operations against ISIS in Syria and Iraq; face down Russian aggression in the Black Sea, and deliver disaster relief to victims of the earthquake in Nepal.

All that’s in addition to their regular duties. On a daily basis, the Navy and Marine Corps already patrol the oceans of the world to protect free trade and counter the threat of rogue nations and terrorists.

The combination of increased daily operations and responding to crisis after crisis, mixed with delayed maintenance and failure to modernize equipment due to a lack of funding, is crippling the Navy and Marine Corps.

Today, sailors and their ships regularly conduct missions that last eight to 10 months. Sailors stay out on patrols longer because replacement ships, aircraft and other equipment are in maintenance backlog.

Navy combatant commanders express fear of “flying the wings off” aircraft that were designed for 6,000 hours of flight and are now expected to fly 10,000 hours because of a lack of replacements.

At this very moment, the aircraft carrier USS Carl Vinson is in the last month of a nine-month marathon deployment, the longest scheduled deployment since the Vietnam War. And, the aircraft carrier USS Dwight D. Eisenhower and its accompanying warships recently completed two back-to-back eight-month missions with only a two-month break in between.

The amphibious warship USS Bataan, which transports Marines and their weapons and equipment to hot spots around the world, completed its second extended mission in 18 months.

These trends cannot continue indefinitely without grave consequences.

In a high-tech world where we’re dazzled by drones and other advanced weaponry, we often forget: The United States is a global economic leader because of the strength of our Navy. Our nation’s economic dependence on the world’s oceans is staggering.

Ninety percent of the world’s commerce travels by sea, including 60% of America’s imports and exports. Ninety-five percent of the world’s intercontinental voice and data traffic travels through undersea cables, including trillions of dollars in banking transactions. Three-quarters of the world’s maritime trade and half of the oil trade pass through a handful of international straits and canals.

Our economic leadership relies on secure, unhindered access to any region of the world that can be reached by ship.

Yet the very success the Navy is having in this essential aspect of its mission is ironically masking the need for increased support that would allow the Navy and Marine Corps, working together, to be as successful in the future as they has been in the past.

We cannot allow this erosion of capabilities to continue. Without a significant investment by Congress and the administration in the Navy’s future, we are placing our national defense, our leadership in the world and the health of both the American and the global economy at great risk.

Lehman was secretary of the Navy in the Reagan administration and a member of the 9/11 Commission.