The U.S. is deploying an additional 1,000 troops to the Middle East in response to "hostile behavior by Iranian forces and their proxy groups" that threaten U.S. "personnel and interests," acting Secretary of Defense Patrick Shanahan announced Monday.

The backdrop: The U.S.-Iran standoff is reaching uncharted waters. As the Trump administration scrambles to rally an international response to Iran’s alleged covert attacks last week, Tehran is taking a long-feared step in broad daylight — announcing it will breach the 2015 nuclear deal’s limits on enriched uranium in 10 day's time.

Shanahan said the U.S. isn't seeking "conflict" with Iran, but would "make adjustments to force levels as necessary given intelligence reporting and credible threats."

Meanwhile, the Pentagon tonight released additional photos it says indicate Iran's Revolutionary Guard Corps was behind attacks on two oil tankers last week.

While the U.K., Saudi Arabia and Israel have backed the administration's assessment, domestic critics and some U.S. allies — including Germany and Japan — have demanded more evidence

The big picture: President Trump now faces dual challenges — deterring attacks on oil shipments through the critical Strait of Hormuz and keeping Iran from marching toward a nuclear weapon — amid a crisis that many of America’s allies privately hold him at least partially responsible for.

Barbara Slavin of the Atlantic Council argues for Axios Expert Voices that Iran is “signaling the death” of the 2015 nuclear deal.

“Even if initial infringements are modest, the combination of rising tensions in the Persian Gulf, a near-total U.S. embargo on Iranian energy exports and Europe’s failure to operationalize a reliable means of trading with Iran is increasingly unstable,” she writes.

“If the Iranians make good on their threat… Tehran will have enough fuel to make a single bomb in less than a year for the first time since the 2015 agreement went into effect,” David Sanger writes in the NY Times.

The White House, meanwhile, is accusing Iran of “nuclear blackmail” and vowing to never allow the regime get a bomb.

The Trump administration argues that the impending crisis only highlights the vulnerabilities of the original deal. Trump’s critics argue that he left the deal without a Plan B, and has simply been squeezing Iran since and waiting for something to burst.

The Trump administration has said its ultimate goal is a new, more comprehensive deal with Iran. Trump has said repeatedly that he doesn’t want a war.

But officials suggested that even absent a deal the maximum pressure campaign would render Iran less dangerous by restricting its cashflow. Recent events are challenging that theory.

What to watch: Iran is now betting that the Trump administration "is too risk-averse to resort to military action and potentially touch off a regional conflagration," Slavin writes. That too will be put to the test.