In the run-up to the NATO summit in Bucharest next month, the Bush administration has launched an intensive diplomatic campaign to persuade our European allies to send additional combat troops to southern Afghanistan, where the Atlantic alliance has been struggling against a resurgent Taliban.

Persuading our reluctant partners to increase their commitments in Afghanistan is important for the sake of the war effort and for the viability of NATO.

The summit also provides an opportunity for the administration to bolster another ally that can provide troops needed to prevail against the Taliban: the armed forces of Afghanistan.

The Afghan National Army is one of the great success stories of the war on terrorism: a multiethnic, increasingly capable professional military force, built from scratch under American tutelage since 2002. It is the most trusted of Afghanistan's fledgling national institutions, commanding the confidence of upwards of 90 percent of Afghans.

The biggest problem is thatthe army is too small, with a targeted end strength of only 80,000 troops. The projected end strength of the Iraqi army is more than 200,000 — even though Afghanistan is nearly 50 percent bigger in territory than Iraq and has a larger population.

Privately, many U.S. officials concede that the Afghan army has nowhere near the necessary numbers to secure its country against an increasingly sophisticated insurgency. The NATO summit is an opportune moment for the U.S. to commit to expanding its ranks in a big way.

I hope President Bush will pledge to support an expansion of the Afghan army, ideally as high as 200,000 soldiers — a bold, new American commitment to reverse Afghanistan's slide toward insecurity and to reinforce our allies there.

The leading argument against a bigger army is cost. Some insist that Afghanistan, one of the world's poorest countries, cannot afford such a large army on its own and that the army must be kept small for "sustainability." This argument is badly flawed.

The United States spends billions of dollars subsidizing the militaries of allies around the world, including many far less strategically important than Afghanistan. Afghan troops are fighting on the front lines against America's mortal enemies. Whatever the cost of ensuring our Afghan allies have the numbers and means to prevail, the cost of their defeat by the Taliban would be infinitely greater.

The costs of a bigger Afghan army should not be borne by American taxpayers alone. Our government should take the lead in establishing an international trust fund to provide long-term financing for the Afghan army. It should also offer our allies — some of whom cannot match America soldier-for-soldier in Afghanistan — the option to make up the gap by funding an equivalent share of the army instead.

Securing Afghanistan with indigenous forces is ultimately less expensive than doing so with foreign troops. For the cost of a single coalition soldier in Afghanistan, we can support 60 to 70 Afghans in uniform.

The other, more challenging obstacle to expansion is the shortage of coalition forces to train and mentor Afghan troops.

But this is not an insuperable problem. The coalition could begin to experiment with thinning its presence inside more battle-hardened Afghan units, which in turn could be partnered with American combat units, as is already done in Iraq. NATO members could also aid this effort by dropping the national caveats that limit the effectiveness of their own embedded trainers inside the Afghan force.

Defeating the Taliban will of course require more than a bigger Afghan army. First, we need to apply the same counterinsurgency principles in southern Afghanistan that have brought so much success in Iraq over the past year — beginning with an integrated, civil-military campaign plan that prioritizes the security of the Afghan people.

As Iraq has also demonstrated, this is a manpower-intensive mission. Increasing the number of coalition troops in Afghanistan can help, but long-term success depends on the emergence of a large number of well-equipped, effective Afghan troops.

The United States has laid the foundation of the indigenous force that can shoulder this fight. What the Afghan army needs now is a surge in support from Washington — a cause that both Democrats and Republicans, united in their desire for victory in Afghanistan, should back.

The Bush administration has an opportunity in Bucharest to provide our allies in Kabul with the military means to prevail in our shared struggle against the forces of extremism and terrorism. It should seize it — and members of both parties in Congress should support it.

Joe Lieberman is an independent Democratic senator from Connecticut. He wrote this commentary for The Washington Post.