is his agency's top priority. welcome to our viewers on pbs in america and around the globe. we start in yemen — where houthi rebels are pressing hard to capture the key city of mareb. it's the last stronghold of the internationally recognised government — and at the heart of yemen's oilfields. the fall of mareb would be a major turning point in the conflict that's been going on for years.saudi arabia — backed by the us and uk — intervened in yemen in 2015 — after the houthis ousted the government from the capital, sanaa.since then — yemen has suffered the world's worst humanitarian crisis. all sides of the conflict have been accused of killing civilians — and other abuses. at least 800,000 people displaced by the war have fled to mareb — and more are on their way.