but just who are those houthi "rebs," and how closely aligned are they to iran? it is that question that has driven american involvement in the war, supporting a saudi-led coalition fighting the rebels. special correspondent jane ferguson smuggled herself across the frontlines in yeme and here, in the third and final report of the series, we look at these important questions behind the world's worst humanitarian disaster. ( chanting ) >> reporter: in the streets of yemen's capital, sana'a, chants of "death to america" are loud d clear. the anger here is runs deep. ( chanting ) these rebels, known as houthis, seized control of sana'a city and much of the north of the country in 2014. they are of yemeni's zaydi sect closest to shiite islam. their growing power caused alarm across the border in sunni saudi arabia, so the saudis formed a coalition of arab countries to defeat them, a coalition backed by the united states. overhe last three years, a campaign of intensive air strikes and a ground war has brought the country to its knees, but hasn't pushed the houthis ba