Breaching the 300-kilogram limit of low-enriched uranium is not in itself a signal that Iran intends to abandon its pledge of peaceful nuclear development. But Iranian officials, in announcing their plans last month, said they were quadrupling production of low-enriched uranium, raising the possibility they could start stockpiling far greater quantities again.

Does this mean Iran is violating the nuclear agreement?

That depends on whom you ask. Iranian officials have said they remain a party to the agreement — unlike the United States. The Iranians have cited language in the agreement, known as the Joint Comprehensive Plan of Action or J.C.P.O.A., in which Iran asserts that if nuclear-related sanctions are reimposed, that would be “grounds to cease performing its commitments under this J.C.P.O.A. in whole or in part.”

Western experts on the agreement have disputed Iran’s reasoning. “It’s basically a unilateral interpretation from the Iranian side of what the nuclear deal means,” said Henry Rome, an Iran analyst at the Eurasia Group, a Washington-based political risk consultancy. “In their view they can reduce compliance commensurate with what the other side does.”

Are other countries violating the accord?

Yes. The United States, by Mr. Trump’s own declaration, is not complying with its provisions, describing the agreement as a disaster that needs to be renegotiated with Iran. The other parties to the agreement — Britain, China, France, Germany and Russia — have all said it was working and have exhorted the United States to rejoin.

Iranian officials have argued that the accord’s European members are not complying with the accord because they have failed to provide the promised economic benefits, fearing American retaliation. The secretary of Iran’s Supreme National Security Council, Ali Shamkhani, said last week that Iran will suspend more of its commitments starting July 7 unless the Europeans stop their “deceitful political attempts aimed at restricting the Iranian people’s obvious rights.”

Mr. Shamkhani did not specify what commitments might be suspended. Other Iranian officials, including President Hassan Rouhani, have hinted at higher uranium enrichment levels, which could reduce the time needed to attain weapons-grade fuel.

Where do the Europeans stand?

European officials, in their most vivid split from Mr. Trump, are scrambling to preserve the agreement, fearful that if it falls apart, the United States and Iran could be headed toward military conflict — and perhaps war.