In the ongoing rise of tension between the US and Iran, which latter includes ties with Russia and the People’s Republic of China, The Wall Street Journal had this bit:

Iran is an important partner for Moscow and one that it wouldn’t want to lose, especially after the US deposed Nicolás Maduro, an ally of Russia, in Venezuela last month. At the same time, Russian President Vladimir Putin isn’t likely to come to the aid of Khamenei if US strikes appear on the verge of bringing him down.
“These relationships are highly pragmatic, highly transactional,” said Alexander Palmer, a fellow with the Center for Strategic and International Studies, a Washington-based think tank, on Tehran’s security ties to Russia and Iran. “They don’t have a sufficient strategic interest in Iran to be willing to go to war with the United States over the country.”

Early in WWII, the UK and Russia invaded Iran and occupied most of the country. The two had agreed to leave Iran six months after the war against Nazi Germany had ended. At the end of those six months, the UK began withdrawing, and Russia indicated it intended to stay for control of Iranian oil. It was only under pressure from the US that the Russians ultimately withdrew.

These days it’s another contest between Russia and the US for controlling influence in Iran. Should the current Iranian government fall, there will ensue two questions: one is whether a stable government be quickly formed and installed. In that event, there likely won’t be much Russia can do, especially as bogged down as the barbarian is in Ukraine.

The other question flows from the political chaos that will develop if no national-capable government is quickly formed. In that event, Russia will move to gain overt control over Iran’s domestic affairs, particularly with regard to Iran’s oil, of which Russia’s primary need is denial of the oil to the world market in order to prop up the prices Russia can get for its own oil, and with regard to Iran’s war materiel production, especially its drone production. From this, the question expands to include the US response to Russia’s moves for control and the contest between Russia and the US over that control.