Since Tehran reached a historic nuclear accord with six world powers in July, there have been a number of incidents that have tested relations with Washington.
The latest, Iran's seizure of two U.S. Navy patrol boats and 10 sailors that had strayed into Iranian waters in the Persian Gulf on January 12, ended amicably. The boats and their crews were released unharmed after Tehran determined that they had not intentionally entered Iranian territory.
But the timing of the episode -- coming just days before international sanctions were expected to be lifted as part of the agreement limiting Iran's nuclear program -- raised hackles among critics who question Tehran's commitment to the deal, and recalled similar flashpoints since it was struck.
Iran created a stir when it conducted two ballistic-missile tests on October 10, firing medium-range surface-to-surface missiles. Iran claims none of its missiles are designed to carry nuclear weapons, and insists that the tests involved conventional missiles meant purely for defense.
An Iranian Emad rocket is test-launched at an undisclosed location in October.
But a panel of United Nations experts concluded that, with a range of at least 1,000 kilometers and a payload of at least 1,000 kilograms, the missiles were indeed capable of delivering nuclear warheads and the tests were a violation of a June 2010 UN Security Council resolution limiting Iranian ballistic-missile tests.
The experts from the UN committee monitoring sanctions against Iran also supported the contention of the United States, France, Britain, and Germany that the tests violated previous UN resolutions -- but not the conditions of the July 2015 nuclear accord.
On November 21, Iran launched another ballistic missile -- raising Western concerns and criticism further.
The missile tests prompted the administration of U.S. President Barack Obama to announce to Congress on December 30 that it was preparing a list of fresh sanctions against Tehran that could be imposed even as other sanctions are lifted under the nuclear deal.
That list reportedly included nearly a dozen defense companies and individuals in Iran, Hong Kong, and the United Arab Emirates that allegedly have supported Iran’s missile program. However, the expected sanctions were subsequently delayed, reportedly because of continuing diplomatic efforts between Washington and Tehran, although a White House official insisted that the imposition of sanctions was not negotiable.
Iranian officials had said that if the United States imposed new sanctions over the missile tests, Iran’s Supreme Leader Ayatollah Ali Khamenei would regard the measures as a violation of the nuclear accord.
President Hassan Rohani, meanwhile, had responded by calling the sanctions threat illegal and ordering his Defense Ministry to accelerate Iran’s ballistic missile program if they were implemented.
Republicans in the U.S. Senate, meanwhile, called on Obama to block the lifting of sanctions under the July nuclear deal.
Rockets In The Strait Of Hormuz
On December 26, Iran fired two rockets within 1.5 kilometers of U.S. and French naval ships that were in international waters of the Strait of Hormuz. The ships -- including the U.S.S. Harry S. Truman aircraft carrier -- were moving from the Arabian Sea into the Persian Gulf to assist an air campaign against Islamic State militants in Syria and Iraq.
A U.S. military spokesman described the altercation as a "highly provocative" incident, saying that it occurred in an area so close to coalition ships and commercial traffic that it could be considered "inconsistent with international maritime law."
The spokesman also said the rocket drill was announced by Iran over maritime radio just 23 minutes before the launch.
Iran's Islamic Revolutionary Guards Corps (IRGC) denied reports of the rocket launches, insisting that no drills had been conducted. General Ramezan Sharif in late December dismissed the reports of the drills as a "psychological operation."
The altercation raised more questions from U.S. politicians, including some Democrats, about Iran's willingness to comply with the July nuclear accord.
Under the terms of the July nuclear deal, Iran agreed to deactivate or remove two-thirds of more than 11,600 centrifuges that produce enriched uranium.
But as that process began in early October, a group of hard-line Iranian lawmakers wrote a letter urging President Rohani to halt the deactivation of centrifuges. They claimed the deactivations were being done too quickly and not according to directives from Supreme Leader Khamenei.
Despite the emergence of a faction of Iranian lawmakers opposed to Tehran's centrifuge promises, the International Atomic Energy Agency (IAEA) confirmed on November 18 that Iran had begun curbing its nuclear program in line with its obligations.
A poster of Supreme Leader Ayatollah Ali Khamenei and the late Ayatollah Ruhollah Khomeini is seen next to a bank of centrifuges in what was described by Iranian state television as a facility in Natanz in 2012.
On December 30, Iranian parliament speaker Ali Larijani announced that Tehran would acquire a new generation of centrifuges in the future that can enhance the "quality of Iran's peaceful nuclear program structure."
Larijani's announcement fueled further criticism of the nuclear deal within the United States by those who allege that Iran is covertly developing nuclear weapons.
Larijani said the acquisition of new centrifuges was more important than the centrifuges Iran currently operates, because the new centrifuges will allow peaceful nuclear research to continue.
U.S. Sailors Captured By Iran
The 10 U.S. sailors and two Navy boats that were detained by Iranian forces in the Persian Gulf near the Iran's Farsi Island on January 12 were initially accused of trespassing, but were held for less than 24 hours.
The sailors were interrogated for hours about their motives, but Iranian officials quickly determined they had not intentionally entered Iranian waters.
In the end, the Pentagon's version of events was confirmed -- one of the U.S. boats had a mechanical failure and drifted into Iranian territorial waters where it and the second boat ran aground.
Iranian boats on January 13 escorted the U.S. sailors and their vessels back to international waters.
In the United States, critics of the nuclear deal with Iran again seized on the incident as evidence of Tehran's ill intentions.
Donald Trump, the frontrunner for the nomination as the Republican Party's presidential candidate, called the incident "an indication of where the hell we are going," adding that the nuclear deal with Iran was "the dumbest deal I've ever seen."
There was no indication from Washington or the government in Tehran that the incident would thwart the implementation of the July nuclear accord. Senior officials in Obama's administration said there was nothing to indicate that the capture of the ships was a hostile act by Iran.
U.S. Secretary of State John Kerry expressed "gratitude to Iranian authorities for their cooperation" in swiftly resolving the issue. He said the peaceful and efficient outcome was "a testament to the critical role diplomacy plays."
Within Iran, analysts say, President Rohani wants to ensure the lifting of sanctions under the nuclear deal in order to bolster the chances of the moderate camp in upcoming parliamentary elections.
With reporting by Reuters, AP, AFP, Fars, and IRNA