Were China to join the Idlib offensive it would allow Beijing to secure the destruction of the Uyghur presence that Turkey helped to foment, while also pressuring Turkey to relinquish the portion of Syrian territory that it is currently occupying.
It is important also to consider that the statement that Gen. Mattis gave is not that he says he disagrees with the Trump decision to launch the Tomahawk strike. He is only saying there is no evidence in his possession that confirms the the Syrian government was behind these attacks.
Secretary of Defense James Mattis stated on or about February 2nd, 2018 that the United States has “no evidence” that the Syrian government used the banned nerve agent Sarin against its own people in attacks in 2013 and 2017. The most recent accusation provoked a massive Tomahawk strike ordered by President