Mark Katz, Professor at the George Mason University Schar School of Policy and Government, the Saudi-Iranian rivalry in the Middle East, which overlaps with the Sunni-Shia divide, makes it very difficult for Russia and the US to resolve issues in the region.
According to Katz, it is not yet clear if the Trump administration prioritizes fighting Iran or al-Qaeda, as there are many other actors, which figures in the administration consider chief threats. Katz ascribed this to Trump's lack of experience in international affairs, which may take time to resolve.