IUNDERSTAND the concerns about sending more troops to Afghanistan. No one wants to put more of our servicemembers in harm's way, or tangled in a difficult and complex conflict. No one wants to be spending more abroad when there's so much to be done at home.

I would not support President Obama's strategy if I didn't believe our mission in Afghanistan is indispensable to America's security and that the cost would be greater - to our security and our treasury - if we left now.

After eight years and significant missteps, the people are justified in their concerns. But the American public should be assured of three things:

This mission is necessary.

The status quo in Afghanistan was sliding backward and would never have produced conditions for us to securely withdraw. Our options were escalate and get the job done, or exit precipitously.

If we were to leave now, Afghanistan would return to the conditions that allowed us to be struck on 9/11. More important, a failed Afghanistan would critically destabilize Pakistan, which currently faces an existential threat from al Qaeda and allied extremists. If Pakistan collapses we could face a situation that is almost unthinkable: a nuclear-armed failed state overrun by the most powerful and most radical jihadist groups in the world.

Al Qaeda is an international organization - we know that and will pursue its operatives wherever they go. But there is nowhere on earth more advantageous to them and more dangerous for the world than where they are right now - in an operational safe haven along the rugged Afghanistan-Pakistan border.

We do not get to pick the battlefield, and leaving the mission unfinished will do nothing to dislodge al Qaeda from its base of support.

Success is attainable.

To eliminate al Qaeda's haven across the border in Pakistan, we must have sufficient forces to lock down the Afghan side and aid Pakistan (with intelligence, drones, training and covert forces) in their efforts. Without our commitment and support, Pakistan will not complete a difficult offensive to root out extremists.

In Afghanistan, our goal is not nation-building, but simply conditions that will be inhospitable to al Qaeda after we depart. Our success cannot and need not be dependent on Afghan forces or the corrupt Karzai government.

The Taliban we face is not the 250,000-man insurrection that defeated the Soviet Union. The Taliban's Afghan forces number only around 20,000, most of them mercenaries. Those fighting for pay or because of political alliances can be brought in from the battlefield. Those ideologically committed (roughly 6,000) can be defeated.

This situation is undoubtedly challenging, but we assess our military strategy based on the facts - not on a "grave of empires" mythology.

The cost is significant - but justified.

The president has provided an honest accounting of the financial cost of this war. This escalation will cost about $30 billion this year. That's high, but would be higher if we muddled along without a sufficient commitment, or worse, had to return.

This should be paid for by reductions in programs elsewhere, closing tax loopholes (including $79 billion for fossil-fuel industries) and/or by new revenue. Unlike the previous administration, we will bring this cost into the budget process and find a way to pay for it without adding to the debt.

My three decades in the Navy have taught me the limits of military force. I opposed the war in Iraq that has made this surge in Afghanistan necessary today. I voted for legislation to require an exit strategy for our forces in Afghanistan and have asked the president to provide clear benchmarks that will trigger our exit if we are successful, or an alternate strategy - including a potentially less effective one of striking at al Qaeda from a distance - if our goals aren't being met.

Based on my experience with Afghanistan as director of the Navy's antiterrorism unit after 9/11, I believe Obama has made the right decision, without regard to political consequences.

I've been outspoken in my support because I believe this mission is necessary, and those who share the president's commitment should stand by him during the toughest decision of his young presidency, the difficulty of which only those who've sent men and women into harm's way can fully understand.

Rep. Joe Sestak, a former admiral and veteran of the war in Afghanistan, is a Democratic candidate for the U.S. Senate from Pennsylvania.