BEIRUT, Lebanon — The Syrian government, flush with pivotal battlefield gains and bolstered by support from Iran and Russia, is finding itself the beneficiary of an evolving regional realignment spurred by the war in Syria.

Egypt and Turkey, countries that were once vocal opponents of Syria’s president, Bashar al-Assad, have, to varying degrees, softened their positions. Egypt, the region’s most populous Sunni country and wary of Iran’s Shiite theocracy, has made its tacit, increasing support of the Syrian government public for the first time. And Turkey, a Sunni regional power, is reshaping the Syrian battlefield by edging closer to Russia and dampening its longtime support for rebels fighting Mr. Assad.

The shifts come at a volatile time as countries in the Middle East long aligned with the United States are hedging their bets and looking to Moscow for support as Russian intervention transforms the conflict in Syria.

The maneuvering comes as Russia asserts itself across the region to a degree not seen since Soviet times, partnering with an increasingly ambitious Iran. Longstanding American alliances with Turkey, Egypt and Saudi Arabia are frayed, and face new uncertainty with the election of Donald J. Trump, whose foreign policy remains largely undefined, except for an avowed eagerness to shake things up.