Early in his first term, President Bush directed that the Defense Department rethink its Cold War-era assumptions and transform itself into an institution capable of addressing the unconventional security challenges of a new century. The president’s vision received added urgency with the attacks of Sept. 11, 2001, and the subsequent campaign against extremism and terror.

In response, defense spending has increased by more than 40 percent since 9/11, not including the additional spending for military operations in Iraq and Afghanistan. The budget the president is proposing for the upcoming fiscal 2006 will continue that spending commitment, with a 4.8 percent increase from last year to an amount exceeding $419 billion.

These increases are needed — and as a nation, we can afford them. Even with the spending increases of the past several years, the United States commits just more than 3 percent of its gross domestic product to defense.

But transforming defense has little to do with how much we spend. More significant than those spending increases has been a notable shift in our priorities — away from an emphasis on size, mass and concentration toward more readily deployable and agile forces that use the latest information technology to operate with greater reach, speed and precision.

The president’s 2006 budget continues this trend. It funds, for example, a next generation Navy destroyer — designed to be manned by a crew of about 100 sailors, as opposed to 300 sailors on current destroyers. It will be able to move into coastal waters and strike targets deep inland.

In addition, not all of these shifts require building new platforms. A 20-year-old Ohio-class Trident ballistic missile submarine — originally conceived as a nuclear deterrent — has been adapted to carry up to 66 Special Forces troops and be capable of launching both cruise missiles and unmanned aerial vehicles. Such a vessel will be put to sea and be prepared to conduct operations this year.

The president’s budget also provides for close to $1.7 billion for the next generation of unmanned aerial vehicles, such as the Predator and Global Hawk — vehicles that have proved so invaluable in operations against terrorists. These aircraft will be bigger, faster and far more lethal in their ability to locate and destroy targets with speed and precision. These advances are made possible by aggressive research and development efforts, which are funded in the new budget with $10.5 billion for science and technology.

Despite the increases in military combat power in each of the services, operations in Afghanistan and Iraq continue to place high demands on the Army to deploy troops with the right skills for today’s unconventional missions.

In response — and under emergency authorities granted by the Congress — the active Army has grown by thousands of soldiers and, by the end of this fiscal year, will be up some 30,000 (not including activated Guard and Reserve forces) from its level four years ago.

This increase in numbers is accompanied, however, by an even greater increase in lethality and capability. The Army is redesigning itself away from large, heavy, Cold War-style divisions to more rapidly deployable and lethal “modular” brigade combat teams. Therefore, an increase in the size of the active Army by less than 10 percent is being matched by an increase in the number of deployable brigade combat teams by approximately 30 percent.

The Special Operations Forces, which have played such a critical role in operations in Iraq and Afghanistan, will grow further in this budget, by 1,400 personnel and four Navy SEAL platoons. The president’s budget also funds new incentives packages to retain the most experienced Special Forces troops.

Operations in Afghanistan, Iraq and elsewhere across the globe highlight the tremendous capability of U.S. forces and the promise shown by speed, lethality and flexibility made possible by new technologies and tactics. As the president begins his second term, we recommit ourselves to questioning old assumptions and outlining new strategies to prepare our forces for the conflict of this century.

Ultimately, though, our country’s security comes down to the courage and resilience of the men and women in uniform, who have performed so heroically and who have sacrificed so much. It is on their behalf that we must continue the drive to transform. The president’s 2006 budget is an important step along that path.

Donald Rumsfeld is the secretary of Defense and a former Republican Congressman from Illinois.