TEHRAN — The punitive American missile strike on Syria for the chemical weapons attack a week ago brought Syria’s most important backers, Russia and Iran, publicly closer together — whether the Iranians want to be or not.

Far from accepting the Trump administration’s version of the chemical weapons assault, the Iranians joined the Russians in rejecting it and doubling down on their expressions of support for President Bashar al-Assad of Syria.

Nonetheless, Iran and Russia do not see eye to eye on everything in a relationship shaped by mistrust, a legacy of Iranian resentment of Russia’s historical expansionism and Soviet-era attempts at domination.

Iranian political analysts say there has been no sign that Iran is prepared to take one key step on Syria: granting Russia’s air force full access to Iranian air bases. Such a move would greatly increase Russia’s firepower and maneuvering space in Syria, and it would pose a new challenge for United States aircraft in the region.