It’s hard to blame President Obama for telling Hamid Karzai that the U.S. is planning to withdraw all its forces from Afghanistan unless Karzai’s successor as president finally signs the Bilateral Security Agreement that Karzai himself negotiated with the U.S. Obama did show a commendable willingness to extend the U.S. troop presence, only to be blindsided by Karzai’s maddening refusal to sign the accord–and by the president’s equally infuriating decision to release dozens of dangerous terrorists from prison.

The good news is that pretty much all of the major candidates running to succeed Karzai have indicated their support for the accord–as has the Loya Jirga that Karzai called to ratify the pact. Ordinary Afghans and especially those serving in the security forces know they need continued U.S. assistance to hold off the Taliban.

And if they needed any reminder of why outside aid is so important, the Taliban’s recent attack on an Afghan army base in volatile Kunar Province provides it. The attackers killed 21 soldiers, who are now being celebrated as “martyrs” across the country. Without a substantial American presence, expect such attacks to become more widespread and deadly, potentially leading to the fracturing of the Afghan security forces and an all-out civil war.

But that presence needs to be more than token or symbolic. Sending just 3,000 troops–one of the four options the White House is reportedly considering–will do little to stabilize Afghanistan. If the U.S. offers only such a puny force, the odds go up that the next president of Afghanistan will take a pass–just as Prime Minister Maliki in Iraq took a pass in 2011 when White House leakers were suggesting the U.S. could keep fewer than 5,000 troops in his country.

To be enticing to the Afghans, a U.S. troop presence has to be large enough–at least 10,000 troops–to make a difference. Obama needs to be careful to get off on the right foot with Karzai’s successor by not making a troop offer that is insultingly and unrealistically small. Indeed, Obama would be well advised to dispel such suspicion now–and to make clear that America will not abandon Afghanistan–by announcing what size force he would like to keep post-2014, assuming the government of Afghanistan agrees.