The Shiite Houthi rebels who descended from northern Yemen and sacked Sana’a late last year have scored a major victory, and in doing so signaled their intent to unite the country under their writ. Taiz, the country’s third-largest city, fell to the Houthis on Sunday. The victory marks another step towards the southern city of Aden, where deposed president Abed Rabbo Mansour Hadi has fled with the remnants of his government.