An article in Breitbart news is a perfect example of why there should be a ban on immigration from Yemen. Multiple terrorist organizations are active in Yemen. On at least two occasions in the last six months missiles were fired from Yemen at US war ships in international waters. Yemen has no record keeping system that could be used to vet refugees, and there is not way to tell a refugee from an enemy combatant. It is not normal to invite the enemy to come live with you.
Al-Qaeda in Yemen Urges Jihadis to Slap ‘New Fool in the White House’ Trump