When Donald J. Trump launched his 2016 presidential campaign, the real estate mogul and reality television star presented himself as a vocal critic of previous American military adventures in the Middle East.

But in that same speech, long before he would start a war with Iran, Mr. Trump opened the door to future conflicts in the Middle East and raised the specter of Islamist terrorism as a national security threat. He pointed chiefly to the Islamic State — which at the time controlled about a third of Syria and 40 percent of Iraq — and to Iran, which had gained significant influence in Iraq by arming militias who were then fighting against the Islamic State.

“Nobody would be tougher on ISIS than Donald Trump,” Mr. Trump vowed.

He warned that “Iran is going to take over the Middle East,” and vowed, “I will stop Iran from getting nuclear weapons,” even as he denounced an agreement negotiated by President Barack Obama aimed at reining in the country’s nuclear weapons program.

This week, Mr. Trump pointed to those decade-old campaign promises as a foundation for his decision to go to war with Iran.

“All I’m doing is keeping my promise,” Mr. Trump said on Monday, adding that Iran “was a threat then and a much bigger threat now.”

Mr. Trump’s remarks, which portrayed victory as just around the corner but vacillated on when and how the war would end, touched on the president’s yearslong campaign of aggression toward Iran’s theocratic government, characterized by escalating military strikes against Iranian forces and their proxies.