When Iran agreed to a deal in 2015 that would require it to surrender 97 percent of the uranium it could use to make nuclear bombs, Russia and China worked alongside the United States and Europe to get the pact done.

The Russians even took Iran’s nuclear fuel, for a hefty fee, prompting celebratory declarations that President Vladimir V. Putin could cooperate with the West on critical security issues and help constrain a disruptive regime in a volatile region.

A lot has changed in the subsequent nine years. China and Russia are now more aligned with Iran’s “Axis of Resistance” to an American-led order, along with the likes of North Korea. When President Biden gathered the leaders of six nations for a video call from the White House on Sunday to plot a common strategy for de-escalating the crisis between Israel and Iran, there was no chance of getting anyone from Beijing or Moscow on the screen.

The disappearance of that unified front is one of the many factors that make this moment seems “particularly dangerous,” said Vali Nasr, an Iranian-born professor at the Johns Hopkins School of Advanced International Studies, “maybe the most dangerous in decades.”

But it is hardly the only one.

President Donald J. Trump’s decision to pull out of the Obama-era nuclear deal triggered a predictable counterreaction from Tehran, and after a long pause, Iran resumed enriching uranium — some to near-bomb-grade quality. Today it is far closer to producing a bomb than it was when the accord was in effect.