Dakota L. Wood

is a senior research fellow specializing in defense programs for the Heritage Foundation's Allison Center for Foreign and National Security Policy and editor of Heritage's "2016 Index of U.S. Military Strength"

The White House has consistently said that it aims to destroy the Islamic State. For more than a year, limited U.S. airstrikes have poked at the terrorist group to little avail. ISIS still controls half of Syria and nearly a third of Iraq and has recently undertaken dramatic attacks against major targets - Hezbollah in Beirut, Russia in Egypt, and the French deep in their capital city - outside the geographic area it controls.

Clearly, the airstrikes have done little to change conditions on the ground. For that, ground forces will be necessary - a force much larger than the 50 special forces operators in Syria and the 200 headed to Iraq, as recently announced by the administration. But President Obama refuses to pursue such an option.

It's not a question of whether the United States has the ability to destroy the Islamic State as a viable military force. America's military could destroy ISIS as a conventional power in weeks. The real question is: "To what end?" What is the geopolitical condition the United States would like to see after such an operation?

Of course, the same questions apply to not taking such action. Are America's security interests better served by allowing the Islamic State to remain in control of its "caliphate," murdering and enslaving with abandon and sowing instability across the Middle East, North Africa, and even Europe? Is it in America's interests to tolerate a terror group that seeks to conduct attacks within our country?

If those are the options - ground operations to destroy ISIS or a tepid approach that leaves it free to export its evil - the ground operation seems the better choice. It would accomplish little long-term good, however, if undertaken without a larger strategic plan. Just consider the awful outcomes of the recent intervention in Libya and our mishandled ventures in Iraq and Afghanistan.

Regimes can be toppled in short order. But without enabling the rise of something better in its place, or at least a structure that maintains order until a long-term solution is found, the reemergence of ISIS (or something much like it) is entirely too predictable.

To preclude that, a military force would need to remain in the war-torn area after the destruction of the Islamic State.

That force could not be solely American. It would have to include ample help - coerced diplomatically and economically, if not willingly provided - from Jordan, Iraq, Saudi Arabia, and Turkey. But these countries certainly won't commit troops without the United States taking the lead.

So let's suppose the U.S. decides it really is important to destroy ISIS and remain as a stabilization force for some lengthy period. What forces would be required, and how might their engagement there affect U.S. security interests elsewhere?

This gets at the heart of strategy and the importance of clearly defining not only the military objective to be achieved but also the political objective it is meant to enable. It also has implications for the capacity to do more than one thing, to handle more than one security interest, at a time.

Estimates of the Islamic State's size vary widely, ranging from 30,000 fighters to as high as 80,000. Reuters suggests 40,000. Any of these numbers seems quite large, but context is important. While ISIS has some anti-armor weapons, mortars, artillery, and even armored vehicles captured from Iraqi and Syrian units, the vast majority of its force is equipped with only a hodgepodge of small arms (pistols and rifles) and various machine guns.

This weaponry does not make ISIS a particularly potent military force. Its victories have come mostly against incompetent forces (such as the Iraqis) that fled the field, small outposts of local militia, or lightly armed and isolated police. Against a highly trained, well-equipped, and comprehensively supported "combined arms maneuver force" such as the U.S. Army would field, it would stand no chance.

Cut off from supplies and reinforcements, with very little awareness of the disposition and maneuver of U.S. forces and lacking any air power at all, Islamic State elements would be faced with three choices: immediate retreat, surrender, or destruction in place.

This isn't to say a U.S. victory would be a cakewalk. Combat of any sort entails risk, especially when fighting in booby-trapped towns and villages and among local populations of uncertain allegiances. But a U.S. military victory would be inevitable.

It's what comes after that should cause concern and at what cost to other U.S. interests.

In the 1991 Gulf War, the United States fielded 700,000 troops to eject Saddam Hussein's army from Kuwait. Drawn from all the services, these forces represented all the combat, combat support, and logistics capabilities needed to prevail against Hussein's well-equipped and entrenched 650,000-man army. Following days of relentless air strikes that "softened" the enemy, the United States won the ground war in 100 hours.

In 2003, the United States and Iraq went at it once again, this time with the United States employing 190,000 against Iraq's 375,000. Later, during the insurgency years, U.S. force levels ranged from 175,000 down to 112,000 against an insurgent force numbering upwards of 70,000. Again, the United States won the military battles.

Against Islamic State, rather poorly armed and questionably supplied, the United States would once again prevail, especially if assisted by 100,000 or more Kurdish peshmerga who would be fighting to secure their homeland in northern Iraq, where ISIS is most concentrated in that country.

Again, it's the "day after" that becomes important.

The United States would likely need no more than five Army brigade combat teams (BCTs) - approximately 25,000 soldiers - plus supporting logistics elements to accomplish the offensive phase. But a similar-size force would be needed to "keep the peace" after ISIS was destroyed. (The chief mistakes made in Libya, Afghanistan, and Iraq had more to do with no involvement after the military phase or trying to remake cultures into something they have never been.)

These four to five BCTs would have to be relieved by a rotational force, as occurred during the occupation of Iraq. A protracted stabilization mission, then, would consume perhaps 20 BCTs with five in each of the following statuses: preparing for deployment, operating in theater, redeploying and recovering, and preparing for the next rotation.

Would the Army be able to support this? Its active component currently stands at 32 BCTs, so dedicating 20 shouldn't be a problem. But perhaps it is more of an issue than one might think. Here is where we get to the current state of the Army and the larger national security interests of the United States.

In 2013, the Army had 45 BCTs. Due to budget cuts it now has 32 and is headed toward 20-something, with an end strength of only 450,000 by the end of 2017 and possibly 420,000 by 2019. Those same budget cuts have also eroded the readiness of Army units. Only a third of the brigades are now rated as meeting acceptable readiness levels.

"Day after" demands would place a tremendous burden on a 32-brigade Army in which less than a third of its troops are deemed ready for action. Worse, if it were tasked with stabilizing the area after destroying ISIS - remember, that's the formal objective - today's smaller, less ready Army would have very little left to handle any other threat challenging U.S. interests elsewhere in the world.

The other services are in a similar situation. The Navy is down to only 272 ships; it's trying to build to 308, but actually needs 350. The Air Force's aging fleet of fighter/attack aircraft has used up approximately 80 percent of its planned life cycle. The Marine Corps needs 36 battalions; for decades it maintained 27; today it has 23.

The trends are all negative, and America's competitors - Russia, China, Iran, North Korea - have taken notice.

Can the United States defeat Islamic State militarily? Yes.

Does it have a plan for what to do afterward? No.

But if it did and committed the forces necessary to enable it, America would be hard-pressed to respond adequately to any other challenges that required significant military power. And that strategic weakness should be the greater concern.

Contact Dakota L. Wood via www.heritage.org.