If you think negotiating over the debt ceiling and the government shutdown is difficult, try sitting across the table from Afghanistan’s President Hamid Karzai. The mercurial leader has routinely trashed his biggest benefactor, gone back on his word, and presided over staggering levels of corruption and political chicanery.

In that respect, Secretary of State John Kerry deserves praise for his effort, during yet another surprise episode of shuttle diplomacy, to salvage a bilateral security agreement that will allow international troops to remain in Afghanistan after 2014, when combat operations led by the North Atlantic Treaty Organization are scheduled to end.

What the “agreement” actually entails and whether it will stick are unclear. One thing, however, is plain. After more than a dozen years, more than 20,000 Americans killed or injured, and more than half a trillion dollars spent, the United States should reject any agreement that doesn’t unequivocally advance its one abiding security interest: to prevent the use of Afghanistan as a base for terrorist attacks against the U.S. and its allies.

Here’s the problem: Neither side has disclosed the agreement’s terms, which cover thorny issues such as U.S. guarantees for Afghanistan’s security, the ability of U.S. forces to operate independently and who has jurisdiction over U.S. force members accused of committing crimes. Karzai has also said that the deal must be approved by Afghanistan’s parliament and a loya jirga, or grand council. And there may be more hurdles. Notwithstanding the Obama administration’s desire to get an agreement wrapped up by the end of this month, the bazaar is still very much open.

It will be interesting to learn more about the “written guarantees” that Karzai said he received from Kerry detailing the U.S. commitment to defend Afghanistan against an outside attack, a particularly ticklish issue given the mutual animosity between Afghanistan and a nuclear-armed Pakistan, another putative U.S. security partner.

As the recent U.S. seizure of a senior Pakistani Taliban leader from Afghan custody shows, any agreement needs to be flexible enough to let the U.S. act independently to pursue terrorists who threaten the U.S.

Although the U.S. has turned over military suspects to civil authorities in Japan and Korea for prosecution, it has good reason not to do the same in Afghanistan.

American diplomats would be wise to make clear to Karzai that any attempt to renegotiate these areas in the draft agreement might prompt the U.S. to abandon it altogether and adopt the so-called zero option of withdrawing troops entirely. The U.S. has better ways to spend $10 billion a year than in defending Afghanistan’s interests at the expense of its own.

Editorial by Bloomberg View