Just how close is Iran to developing a usable nuclear weapon? That question is at the heart of the Iran-Israel conflict -- and one that's driving how President Donald Trump responds -- but Iran's nuclear capabilities are still very much shrouded in mystery.

When Israel launched its series of strikes against Iran last week, it also issued a number of dire warnings about the country’s nuclear program, suggesting Iran was fast approaching a point of no return in its quest to obtain nuclear weapons and that the strikes were necessary to preempt that outcome.

President Donald Trump in about eight hours went from suggesting a nuclear deal with Iran remained “achievable” to urging Tehran’s 9.5 million residents to flee for their lives as he cut short his visit to an international summit to return to Washington for urgent talks with his national security team.