The sanctions end all the economic benefits America granted Tehran for its 2015 nuclear deal with world powers, though Iran for now continues to abide by the accord that saw it limit its enrichment of uranium. While for now not threatening to resume higher enrichment, Iranian officials in recent months have made a point to threaten that could resume at any time faster than before.
The new American sanctions particularly hurt Iran's vital oil industry, a crucial source of hard currency for its anemic economy. Its national currency has plummeted over the last year, sending prices for everything from mobile phones to medicine skyrocketing.
"Today, Iran is able to sell its oil and it will sell," Rouhani vowed Monday as the sanctions kicked in.
Iranian state television aired footage of air defense systems and anti-aircraft batteries in two-day military maneuvers underway across a vast stretch of the country's north.
The drill was to continue through Tuesday. Iranian army Gen. Habibillah Sayyari said both the national army and the country's paramilitary Revolutionary Guard were taking part in the exercise.
Rouhani, meanwhile, pledged to government officials in comments aired on state TV that Iran would overcome the sanctions.
"We are in the war situation, " Rouhani said. "We are in the economic war situation. We are confronting a bullying enemy. We have to stand to win."
Iran is already in the grip of an economic crisis. Its national currency, the rial, now trades at 145,000 to one U.S. dollar, down from when it traded 40,500 to $1 a year ago. The economic chaos sparked mass anti-government protests at the end of last year which resulted in nearly 5,000 reported arrests and at least 25 people being killed. Sporadic demonstrations still continue.
The United States says the sanctions are not aimed at toppling the government, but at persuading it to radically change its policies, including its support for regional militant groups and its development of long-range ballistic missiles.
However, President Donald Trump's personal lawyer Rudy Giuliani and John Bolton, the president's national security adviser, both have made public statements supporting overthrowing Iran's theocratic government.