Saudi forces are fighting Houthi rebels in which neighbouring country?

Yemen, one of the world's poorest countries and a crucial US ally in Washington's fight against al-Qaeda, is in the midst of a series of conflicts that threaten its stability. The government has for five-years waged a campaign against a group of Shia Muslim fighters in the country's north. The conflict with the Houthi fighters, which has cost the lives of thousands of people, is a mix of local and tribal concerns stemming from historical roots. Saudi Arabia, for its part, worries that strife instigated by the Shia sect so close to Yemen's border with Saudi Arabia could stir up groups in Saudi itself.