Muslims have been fighting and killing each other since the early days of Islam. It was a convert from Judaism who caused a split by proclaiming that Ali is God (much as Christians said the same thing about Jesus). Then Islam spread both towards the East and the West, with Iran's Muslims following the Shia version. I remember the Shah of Iran saying, "We can't stay idle while hundreds of Shias are being killed in Yemen". The two sects are now different religions, each calling their version Islam.
So it's no surprise that the Houthis of Yemen (who are Shias) have entered the war and attacked Saudi Arabia. There is a widespread belief that the US and Israel have told them to do so. In just a couple of days, the poor Saudis have been reduced to begging the US and Israel for help. Perhaps Pakistan may also be trapped into taking sides and sending troops to the war torn region. It is, of course, the Saudi's fault. They should have spent more on increasing the size of their army instead of buying paintings costing fifty million dollars.
It looks like the beginning of a world war.