Dear Sen. John McCain:

You know as well as anyone, the Department of Defense and the US military are at greater risk today than at any time since Sept. 11. The principal threat is not al-Qaida, Da'esh or a resurgent China or Russia, although a good part of the world remains dangerously unstable. Nor is sequestration, despite its irrationality, the mortal danger, although it will harm the forces.

Instead, the Pentagon faces three internal threats that can be characterized in terms of strategy, people and uncontrollable cost growth. If not addressed, the predictable outcome will be a "hollow force," reminiscent of what happened after the Vietnam War when our military capability, preparedness and morale imploded.

As chairman of the Armed Services Committee and the US Congress' top defense expert, you are one of the very few who understand and can prevent a 21st century equivalent of the hollow force of the 1970s and early 1980s.

Every administration going back to George Washington probably has been criticized for having no strategy or the wrong strategy. Today is no different. The fundamental flaw is a vapid national security strategy based more on grand aspirations, rhetoric and slogans rather than on well-defined objectives, priorities and a plan to achieve them.

As the nation recovers from nearly a decade-and-a-half of war with constant combat deployments in which few expenses were spared, the military will have to adapt to more mundane peacetime roles and constraints. Preparing for conflict during peace is far different than preparing to deploy to war because going in harm's way tends to focus the mind rather sharply.

Readiness will suffer. The forces will not be equipped as well as they were. And the military will have to take up tasks once filled by civilian contractors from standing gate guard duty to food preparation to mowing grass.

For the ground forces, specifically trained for capture or kill missions against insurgents rather than fighting other armies, a transition will be required. But the resources for retraining are unlikely to be available. As a result, a significant portion of the force who were highly motivated by serving their country in combat will chafe at peacetime duties.

But the most diabolical threat and indeed cancer is exploding internal cost growth for pay, allowances, health care, retirees, weapons systems, operations, training, overhead and many other items. That cost growth is about 4-7 percent annually above inflation.

Studies and warnings issued by the Defense Business Board document these trends and the consequences. If action is not taken now to arrest this cost growth, including cutting overhead that must include unneeded bases and infrastructure, by decade's end we will be well on our way to a hollow-like force even if sequestration is abandoned and defense spending kept at current levels.

These internal threats are not insurmountable. My view is that the nation will be more than protected if it has the capacity to have deployed and be able to deploy a joint ready force of 300,000 nominally divided between both coasts. That would in turn require a total force of under 1 million with obvious changes to the reserve and National Guard assuming that force is well-trained and equipped. Indeed, it is difficult to envisage where the US would need to deploy forces of that size unless we attacked China or Iran.

Regarding people, the Pentagon must recognize this crisis in transitioning the forces to a post-Iraq and Afghanistan world even though some number will be deployed in harm's way. However the greatest threat is cost growth. That must be attacked now.

Your committee needs to take the lead in this. Congress will not be persuaded to cut pay and benefits or bases and defense industries in their districts. Yet the choice will be that or a hollow force made worse when interest rates rise, which they must, increasing the cost of servicing nearly a $20 trillion national debt.

Straight talk is needed. We have the option of having an oversized, unready military or a slightly smaller, highly capable and motivated force that can be supported by likely future budgets.

While nothing is simple, the choice is stark, real and looming. Over to you, Sen. McCain.

Ullman is the chairman of the Killowen Group, which advises government and business leaders, and senior adviser at Washington's Atlantic Council and Business Executives for National Security.

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