This is a moment many thought would never come: Iran has delivered on its commitment under a 2015 agreement with the United States and other major powers to curb or eliminate the most dangerous elements of its nuclear program. The world is now safer for this.

The International Atomic Energy Agency verified on Saturday that Iran has shipped over 8.5 tons of enriched uranium to Russia so Iran can’t use that in bomb-making, disabled more than 12,000 centrifuges and poured concrete into the core of a reactor at Arak designed to produce plutonium.

On Sunday, President Obama hailed these steps as having “cut off every single path Iran could have used to build a bomb” and noted that engagement with Iran has created a “window to try to resolve important issues.” Most important of all, he said, “We’ve achieved this historic progress through diplomacy, without resorting to another war in the Middle East.”

Still, there are daunting challenges ahead, including ensuring the deal is strictly adhered to, an obligation for the United States, Russia, China and Europe. Cheating should be much harder, given that Iran will be subjected to continuous and intrusive monitoring by the I.A.E.A. of its nuclear enrichment facilities, centrifuge production and uranium mines. And even if the Iranians were to attempt to produce enough nuclear fuel for a bomb, it will now take them more than a year to do so. Before the agreement, that breakout time was two to three months.