 Complete the following phrase:  Yemen has been called one of the major fronts in the conflict as a result of the revolution and subsequent civil war. Yemen had for years been within the Saudi sphere of influence. The decade-long Houthi insurgency in Yemen stoked tensions with Iran, with accusations of covert support for the rebels. A 2015 UN report alleged that Iran provided the Houthi rebels with money, training, and arms shipments beginning in 2009. However, the degree of support has been subject to debate, and accusations of greater involvement have been denied by Iran. The 