"Yes. They reside along the Empire's northern border, and are one of Equestria's oldest allies." Luna replied. "Our alliance goes back over a thousand years, shortly after the war between Equestria and the Crystal Empire began. The Yak's leader, Chief Erik, was afraid that Sombra would attempt to invade the much smaller confederation. So, despite plenty of internal protests, he signed an alliance with Equestria. Sombra wasn't expecting the famously isolationist Yaks to get involved, so he didn't even station troops along the border. That was one of many mistakes he made that led to his downfall."