And the control of a key border crossing with Turkey
The question of which is the strongest rebel group in Syria has been answered. It's Syrian al-Qaeda or Hayat Tahrir al-Sham (HTS).
The two largest Syrian rebel groups, Tahrir al-Sham and Ahrar al-Sham have clashed, and continue to clash, in numerous places in the main rebel-held territory in Idlib province in north-western Syria.
Additionally, everywhere Tahrir al-Sham is scoring decisive victories with Ahrar forced to retreat. Over the weekend HTS wrestled full control of the previously divided rebel capital, Idlib. HTS also wrestled control of the key Bab al-Hawa border crossing with Turkey, and forced a number of Ahrar companies in less important locations to flee to Turkey or switch allegiances.
This isn't so much a fight to the death as a clash to show who is boss. But if so HTS has staked a very convincing claim as the only viable leader of Syria's Islamist rebellion.
Up until 2016 Ahrar and HTS (then al-Nusra) were extremely close, with Ahrar serving as the slightly more respectable advocate for the HTS. Since then they've both united numerous other groups in two rival coalitions behind them. The two coalitions are now vying for overall primacy, but the coalition around al-Qaeda is now winning that contest handily.
The contradictions between the two stem from the fact that on Turkey's insistence, unlike HTS, Ahrar was not categorized as a terrorist organization by the Russian-led Astana peace process.
Cut off from Turkish aid which still flow to Ahrar and finding itself singled out as the only rebel outfit marked for destruction HTS naturally needs to make sure it does not actually find itself isolated from the rest of rebellion—and the surest way of doing that is if can crack down on other rebel groups and force them into a subservient, satellite position.
If HTS succeeds at that, that will make the Astana de-escalation plan null and void and spell end the of cease fire for Idlib. But if by that time ISIS in eastern Syria has been broken and the Syrian army is free to concentrate on the west, Damascus, if not Russia and Turkey, might welcome the chance to bury Astana and resolve things militarily.
HTS and Ahrar differ in terms of ideology only in the point that Ahrar denies any international ambition. Unlike al-Qaeda, it claims it would be satisfied with a Salafist theocracy in Syria and would not seek a globe-spanning Caliphate.