Shiite rebels and Sunni militiamen battled in Sanaa for a second day Friday in battles that have killed at least 120 people and have shaken the Yemeni capital with thousands fleeing their homes. The violence raises fears that this chronically unstable country could be dragged into the sort of sectarian conflicts that have plagued other nations in the region.

Yemen has had years of turmoil and division, particularly a longtime battle with perhaps the most dangerous branch of the al-Qaida terror network, separatist uprisings in the south and political upheaval that overthrew a longtime autocrat, all on top of deep poverty and tribal tensions.

But throughout, it had largely been spared Shiite-Sunni hatreds like those that tore apart Syria and Iraq. Just under half the population is Shiite, but they almost all belong to a unique version of Shiism — Zaydi — which is seen as very close to Sunni Islam. The two communities have long been intertwined in the political elite and military. For example, the former president, Ali Abdullah Saleh, ousted in 2012, was a Zaydi but most of his political alliances were with Sunnis. […]