The Great Powers contend for control of Syria because it is the path for transferring natural gas from the Pas field, the world’s largest deposit of natural gas, from the Persian Gulf towrope. Iran an Qatar are the main contenders because they share the Pars field.
Russia wants to prevent Qatar from selling to Europe because that would undercut Russia’s income from the sale of crude to Europe, harming its economy and, more importunately, its people. (Natural gas is cleaner and would sell better in Europe than crude). Russia is not interested in the Islamic State; it is interested in Syria's having a government opposed to the Persian Gulf dictators, Oman excepted.
Iran, which has half the Pars field, also wants to sell to Europe. Russia thinks it can control the price Iran would sell and so would not hurt its market.
The Saudi want to control Syria because it sees an opportunity to turn at least one country into another Wahhabist state, something it has tried for decades to do with no success because no people want to live to under Wahhabismic rule.
The US and Britain support the Saudi and oppose Iran for historical reasons. The time for that position is past. The West should as quickly as possible switch positions and support Iran.
The switch cannot be quick in the US, because of the millions of dollars the Saudi spend lobbying Congress, but it can be done over time. Weakening Saudi control over the price of oil would help the US economy. That is in its national interest.
The Saudi lobby is linked with the Israeli lobby, making the change even more difficult. Regime change in Israel should be a prime objective of Western governments. Now that China is supporting Palestinians, is to past time for us to be on Palestine's side. Israel, too, needs to be friends with Palestine if it hopes to remain a democratic state instead of a totalitarian one. It is in American Jews’ interest that Israel be aa democracy.
An immediate step the US and Britain should take is to withdraw all support for Saudi Arabia's genetical bombing o Yemen.