The Gulf states have moved quickly to aid and invest in Syria after the sudden collapse of the country’s dictatorship, sensing a new opportunity to expand their influence in the Middle East.

The ousted president of Syria, Bashar al-Assad, was long under the sway of Iran, the Gulf’s traditional regional rival. After a coalition of rebels surprisingly unseated him in December, it was officials and businesspeople from Gulf nations who stepped up first to help Syria begin rebuilding.

When President Trump announced in May — during a state visit to Saudi Arabia — that he would suspend American sanctions on Syria, it gave both an incentive and an assurance for investors to do even more.

Syria, now led by a former rebel fighter turned president, Ahmed al-Shara, is at a “turning point,” as is the rest of the Middle East, Prince Faisal bin Farhan, the Saudi foreign minister, said in May. He was speaking after Mr. al-Shara met with Mr. Trump in the Saudi capital, Riyadh.

“We are in an incredibly difficult and dangerous time in the region — but nonetheless we are also in a time that offers incredible opportunity,” he added. “If we can now all come together to support the Syrian people and the Syrian government in actually taking charge, and building a prosperous stable secure Syria, that will impact all of the region.”

Qatar and Saudi Arabia have paid off Syria’s $15.5 million World Bank debt, making the country eligible for grants to fund reconstruction after a devastating civil war that lasted almost 14 years.