Bruce Riedel comments  on how the war on Yemen has diverted Saudi and GCC resources and attention from the war on ISIS. He also notes that the war has strengthened jihadists and unwittingly aided Iran:
Even worse is that the major beneficiaries of the war so far are al-Qaeda and Iran. Al-Qaeda has seized control of large parts of southeastern Yemen since the war began. Its black flags fly in Aden, the temporary capital of the pro-Saudi government. Al-Qaeda in the Arabian Peninsula has grown stronger in the months since it attacked Paris in January, not weaker. That is a disturbing portent for those now promising to defeat IS.
Iran is fighting to the last Houthi, laughing at the Saudis and Emiratis as they spend resources in what Tehran hopes will be an endless quagmire. Iran gains in Iraq and Syria from the Sunni forces’ diversion to Yemen.
As I’ve said before, the Saudi-led intervention has been failing on the Saudis’ own terms. The coalition hasn’t been able to drive the Houthis out of the capital, nor does it seem likely to be able to do so anytime soon. The Saudis haven’t been able to reimpose Hadi as president, part of the country is ruled by AQAP, and the territory that the coalition nominally controls is vulnerable to attacks from them and the local ISIS affiliate. Instead of weakening Iranian influence in the region, they opted to attack a country where that influence was minimal, and so they are fighting a proxy war with Iran in a place where they can do the least harm to Iran. If one wanted to counter Iranian influence in the region, waging a protracted, atrocious war in Yemen would be the most useless thing that one could do, so of course this is what the Saudis and their allies have been doing for the last eight months with U.S. and British backing.
On top of all that, an entire country has been devastated and its people have been starved of basic necessities to such an extent that millions of people are at risk of famine. None of the countries involved in battering and starving Yemen has been made more secure, Yemen has been wrecked, and all so that our panicked despotic clients could pretend that they were dealing a blow to Iranian “expansionism” that wasn’t even happening. This is the disgraceful and unnecessary war that the U.S. has supported from the beginning for the sake of “reassuring” those despots that they can count on us to indulge their most stupid and destructive behavior.