Matthew Kroenig, who served as special adviser on Iran to the Office of the Secretary of Defense between July 2010 and July 2011, offered some of the possible “red lines” for a military strike in a recent Foreign Affairs article he wrote. He argued that the U.S should attack Iran’s facilities if Iran expels international nuclear weapons inspectors, begins enriching its stockpiles of uranium to weapons-grade levels of 90 percent, or installs advanced centrifuges at its main uranium-enrichment facility in Qom.

In an interview with The Daily Beast, Kroenig also noted that Iran announced in 2009 that it was set to construct 10 new uranium enrichment sites. “I doubt they are building ten new sites, but I would be surprised if Iran was not racing to build some secret enrichment facilities,” Kroenig said. “Progress on new facilities would be a major factor in our assessment of Iran’s nuclear program and shape all aspects of our policy towards this including the decision to use force.”

Until recently, current and former Obama administration officials would barely broach the topic in public, only hinting vaguely that all options are on the table to stop Iran’s program. Part of the reason for this was that Obama came into office committed to pursuing negotiations with Iran. When the diplomatic approach petered out, the White House began building international and economic pressure on Iran, often in close coordination with Israel.