DOHA, Qatar — Global anxiety that the United States will take military action against Iran has increased now that President Trump has appointed John Bolton as his national security adviser. Mr. Bolton has long promoted regime change in Iran, argued for bombing Iran and a more assertive American policy against Iranian expansionism in the Middle East.

But the United States cannot effectively confront Tehran and its proxies until it appreciates Iran’s role in state building in Middle Eastern countries decimated by conflict.

Iran has increased its influence in the region since the eruption of the Syrian civil war and the rise of the Islamic State. Iran mobilized tens of thousands of Hezbollah fighters and other Shiite militias from Iraq, Pakistan and Afghanistan to fight alongside the forces of the Syrian president, Bashar al-Assad. These militias played a critical role in defeating the Syrian rebel groups. They also fought against the Islamic State in Syria and Iraq, at times in close proximity to American forces.

Iraqi Shiite militias, battle-hardened from fighting the United States, began fighting in Syria alongside Mr. Assad’s forces in 2012. Hezbollah captured the strategic Syrian town of Qusair from opposition fighters in 2013. Shiite militias, including Afghan fighters, were pivotal in capturing Aleppo in December 2016, which arguably secured the survival of the Assad regime.