This is the first in an unprecedented series of four opinion pieces about the future of the Air Force penned by its most senior civilian, Air Force Secretary Michael Donley.

In more than 15 years covering the US military, I don’t remember a senior Pentagon official penning a series like this, and we are honored to run it. The series is, I think, an indication of just how deeply worried senior defense officials are about the future. Sequestration isn’t really fixed, despite last week’s momentary spasm of rationality on Capitol Hill. Defense budgets are likely to continue dropping over the next five years at a time when America faces enormous and widespread national security challenges – Iran, Syria, North Korea, a wobbly European Union, China, global warming, Al Qaeda and its friends – and those are a few of the ones we know about.

If there is one theme to Donley’s op-eds, it is this: We are smaller and likely to get even smaller at a time when our weapons are old and we are trying to replace them. America’s leaders, its people, and our allies depend on the United States Air Force for control of air and space, gathering intelligence, moving people and equipment anywhere on short notice, and the ability to let bad guys know that, no matter where in the world they are, we have weapons that can do them harm. Crafting the right mix of people and weapons to accomplish all those things in these dangerous days is going to be incredibly challenging.

In these op-eds, Donley grapples with, among other issues: the difficulties of convincing Congress to let him retire planes and find the right balance of active, Reserve, and (most controversially) Air National Guard forces; how to replace an aircraft fleet that is, on average, 24 years old without breaking the bank while still maintaining the ability to be almost everywhere and able to do whatever is needed; and how to keep planes and people in the air, ready to fly and, if necessary, fight. We will run one op-ed each day through Friday. The Editor

Since coming to Washington in 1978, I watched from vantage points including Capitol Hill, the White House, and the Pentagon as the defense budget rose dramatically during the Reagan buildup and then declined after Operation Desert Storm as part of the post-Cold War “peace dividend.” Now the cycle is repeating again as higher post-9/11 defense budgets driven by operations in Iraq and Afghanistan begin to recede and our nation focuses on getting its fiscal house in order.

While still supporting ongoing combat operations in Afghanistan, confronting immediate security challenges throughout the greater Middle East, and putting greater focus on the Pacific, we ask ourselves: How should the Department of Defense balance competing defense needs among the size of our force structure, today’s readiness and modernization for the future?

From our collective experience in the 1970s, the generation of defense leaders with whom I serve learned that during periods of fiscal austerity, tough decisions have to be made to avoid a hollow military. I define this as one that looks good on paper, but has more units and equipment than it can support, lacks the resources to adequately man, train and maintain them, or keep up with advancing technologies.

Confronted today by a more complex and dynamic security environment, as well as a significant reduction in defense resources, Air Force leadership determined the best path forward is to become smaller in order to protect a high quality and ready force that will improve in capability over time.

In devising our fiscal 2013 defense budget and planning for the years after, we decided we must get smaller to ensure a fully trained and ready force that maintains the scope of capabilities and flexibility to engage a full range of contingencies and threats. The 2011 Libya operation reminded us that in today’s security environment the Air Force must be ready to respond to rapidly emerging crises. We simply do not have months to prepare or to rebuild the readiness of an unready force.

Neither can we assume that when called we will operate only in a benign, uncontested, or low threat environment. In some situations, even performing non-hostile or humanitarian missions can involve significant risks, so we must always be prepared to operate and prevail in places that are well-defended. In addition, the US often leads international coalitions who rely on us to facilitate unity of effort and to backstop their more limited capabilities. Mitigating the risk associated with a smaller military thus requires a ready force, versatile and effective across a broad spectrum of potential contingencies.

Improving capabilities over time is also critical to our future and to maintaining the qualitative advantage on which our nation’s security has depended for decades. Even as the defense budget comes down from its post-9/11 peak and our forces become smaller, it is nonetheless essential to make room for modernization, which accounts for about 30 percent of the Air Force budget. Most of our fighters and helicopters were built in the 1980s. Nearly half of our bombers and most of our aerial refueling tankers were built in the 1950s and 1960s, when the United States spent around 8 percent of its GDP on defense.

Today our nation spends closer to 4 percent of GDP on defense and the Air Force now finds itself with a geriatric fleet on average more than 24 years old – with too many planes approaching 50 years old. Since 9/11, the Air Force built out the C-17 airlift fleet, bought 187 stealthy F-22s and nearly 300 Remotely Piloted Aircraft. But RPAs built for Iraq and Afghanistan may not survive in contested airspace. Key satellites are approaching the end of their expected service life. Aging components in our nuclear and communication enterprises face obsolescence and still await refreshment from the latest generations of information technology.

In addition, the missions and technological threats confronting our military continue to change. Modern air defenses, missile defense, congested and contested operations in space, and cyber defense are among the rapidly evolving challenges that also require new investment. The globalization of information technologies continues to fuel advanced military research and development abroad. Consequently, in some areas the US is working harder to sustain more narrow military advantages. Between aging inventories and new, more sophisticated threats, this is clearly not the time for a procurement holiday.

So, how to get smaller? And how small is too small? The Air Force has retired nearly 1,900 aircraft and downsized by over 30,000 active duty personnel in the last decade. In planning for a yet smaller force, our decisions have favored keeping aircraft and equipment that can be used for many purposes over those with more narrowly focused capabilities. Where feasible, we seek to divest smaller fleets with niche capabilities, and also focus on common versions of key aircraft to maximize operational flexibility and minimize sustainment costs.

Accordingly, the FY13 budget request identified 286 aircraft for elimination across the Total Force – Active, Guard, and Reserve – over the next five years. These changes would have resulted in a reduction of 9,900 Total Force Airmen during FY13. These reductions, however, were widely criticized as falling too heavily on the Guard and Reserve.

There is little disagreement that the Air Force must maintain readiness and modernize; however, there is real resistance to divesting aircraft and downsizing installations. It is understandably difficult to accept reductions that affect individual communities, an obvious reason why Congress has thus far been reluctant to approve another round of base closures. But it is not possible to avoid real impacts when programming the $487 billion in defense reductions required by the Budget Control Act.

Over the past few months, Air Force active duty, Guard and Reserve leadership has come together to rebalance the reductions across our Total Force. Our revised, compromise plan recently approved by Congress restores about 38 percent of the aircraft and 55 percent of the personnel reductions originally proposed for the Guard and Reserve. At the same time, it permits the Air Force to proceed with selected aircraft retirements and transfers necessary to meet budget targets while protecting readiness and modernization.

With these changes, the active duty Air Force will be down to approximately 329,000 personnel – approaching the same size as when it was established as a separate service in 1947. There are real questions about how much smaller the Air Force can become without incurring significant risk to the capabilities we provide to joint and coalition forces, including control of air and space, gathering intelligence around the world, moving people and equipment anywhere on short notice, and the ability to hold any target at risk.

Like the other services, the Air Force will work with our defense and national leadership to fine tune our plans and programs as we confront both a dynamic security environment and the nation’s fiscal challenges. We will adjust and compromise as necessary, but we’ll need broad consensus with Congress on the way forward to avoid a hollow force. Trading size to maintain a quality force, and staying focused on readiness and modernization, will be politically difficult and challenging to implement. But absent additional resources, this likely remains the best combination of choices available to sustain America’s military as the world’s finest.