(Corrects report’s conclusion in second paragraph.)

A decade ago, weak evidence of the crimes of Saddam Hussein helped lead the U.S. and its allies to war in Iraq. Today, strong evidence of war crimes by Bashar al-Assad may help the U.S. and its allies avoid war in Syria.

The damning United Nations report confirming the use of chemical weapons also highlights the importance of the Sept. 14 agreement putting Syria’s chemical weapons on the path to destruction -- and the huge challenges of doing so. It also points to the need to leverage this agreement into a lasting political solution to Syria’s increasingly horrific conflict.

The UN investigators say they have “clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin” were used to attack several neighborhoods. To reach that conclusion, they interviewed 50 survivors and collected 30 soil and surface samples from impact sites, as well as blood, urine and hair from 34 affected residents. More than 85 percent of the blood samples tested positive for sarin.

Such clinical specificity on the part of the UN may be reassuring to those not moved by the Barack Obama administration’s more general public assertions (or, for that matter, to those still dubious of any U.S. claims about weapons of mass destruction). More important, such results underscore the significance of an agreement that promises to spare Syrians more such horrors, destroy a potential source of chemical weapons for terrorists, and reduce the number of countries that have failed to sign the Chemical Weapons Convention from five to four.

If Syria abides by the terms of the pact reached by U.S. Secretary of State John Kerry and Russian Foreign Secretary Sergei Lavrov, both it and the world will be a better place. That’s a big if. The UN team’s efforts were hampered by intimidation, violence and the generalized chaos of operating in a war zone. Never mind the Syrian regime’s apparent effort to destroy evidence of the chemical assault: Exhibit A of its deep-seated duplicity is its sudden willingness to acknowledge and give up an arsenal that it had denied having in the first place.

The Sept. 14 agreement sets tight deadlines: Syria has one week to submit an inventory of its chemical weapons, with initial inspections by November, and must carry out the complete removal and destruction of its chemical weapons by the first half of 2014. Yet while the U.S.-Russia agreement grants inspectors the “immediate and unfettered right to inspect any and all sites in Syria,” what happens after Oct. 14, when Syria formally becomes a member of the Chemical Weapons Convention and, with Russia, has the right to try to block “challenge inspections”? Ironically, national exemptions carved out by U.S. legislators during the late 1990s may end up making it easier for Assad to do just that.

These are just some of the thorny issues that the U.S. and Russia must work out as they negotiate procedures for destroying Syria’s chemical weapons in a country at war, along with a UN Security Council resolution setting out the consequences for any Syrian backsliding. And for all the complaints about Obama’s tortuous negotiating style -- complaints that the president has tartly dismissed -- one thing is clear: It was the U.S.’s threat of military force that brought us to this hopeful point, and it must be retained.

The UN report also makes something else clear: the need to redouble efforts for a political settlement, and to write a future for Syria that does not include Assad. In keeping with its limited mandate, the report offers no “smoking missile,” but the circumstantial evidence confirming earlier U.S. assertions about the regime’s culpability is compelling, as is UN Secretary-General Ban Ki-Moon’s statement that “this act is a war crime.”

It’s far too early to say that this agreement lays out a path for a larger solution. But it does show what the U.S. and Russia can achieve when, whether for the wrong or the right reasons, their interests align.

To contact the Bloomberg View editorial board: view@bloomberg.net.