The inevitable backlash came from the Zaydi Houthi tribal areas in the mountainous north of Yemen, and in 2015 the Houthis over-ran Sana'a.
The Republic of Yemen government fled south to Aden on the Arabian Sea coast and Yemen - last united in 1990 - returned to being two countries. The Republic holds South Yemen (population eight million) which is larger in area than North Yemen (population 21 million) but much of it is desert.
Back during the Cold War, the Soviet Union supported South Yemen and the Americans backed North Yemen. Now the situation has reversed and there are US special forces in the south supporting the Republic of Yemen in exile and co-ordinating the war against the Houthis, who are supported by Iran.
Osama bin Laden was a Sunni Saudi of Yemeni descent, and a Wahabi, and al Qaeda in the Yemen also supports the Republic of Yemen.
The country's late president Ali Abdullah Saleh once said: "Governing Yemen is like dancing on the heads of snakes."
North Yemen, currently held by the Houthis, can be further divided along altitude lines - between the Red Sea coast where food is imported and the highlands where the capital Sana'a sits.
Yemen does not grow enough food to feed itself and the Saudis have destroyed the Red Sea port infrastructure, blockading food imports. Yemen could possibly grow enough if they grew lentils instead of khat, the narcotic leaves of which are chewed as a stimulant.
Since the 1960s, US arms manufacturers have lobbied their government representatives about capitalising on the conflicts in the Middle East - it would be better for American businesses if these wars were fought with US arms, not those made by other members of the P5 (France, Britain, Russia and China) they reasoned.
In 2009, Saudi Arabia started fighting Yemen's Houthis with arms supplied by the Obama administration.
Saudi Arabia is reluctant to put soldiers into Yemen and has conducted a "smart war" from the air.
Drones and jet fighters with "dumb bombs with graduate degree guidance systems" were deployed by Riyadh - with American technical assistance - and Yemen descended into an Islamic blood feud, half the casualties of which were civilian. After selling the Saudis $100 billion worth of the latest in American weapons systems, and after the Saudis had killed the neutral mayor of Sana'a, with a guided missile at a funeral, the Obama administration started raising concerns about civilian casualties with the Saudis.
Barack Obama's signing of the "nuclear non-proliferation" treaty with Iran further angered the Saudis.
Incoming US president Donald Trump "decoupled humanitarian from security concerns" in Yemen and last year he signed a deal for another US$110 billion ($160 billion) of arms sales to Saudi Arabia.
He said the deal represented "hundreds of billions of dollars of investment in the US and jobs, jobs, jobs."
Trump's simplistic policy in the Middle East is to set Sunni and Shi'a Muslims against one another and then sell arms to create US jobs and pay for US oil imports.
Peace would be bad for the American economy.
In 2010, when Whanganui couple Doug and Marion Davidson visited Yemen, the Houthis had yet to reach Sana'a.
At night the stained glass windows of Sana'a's Unesco World Heritage old town looked "like something out of the Arabian Nights", Marion said.
■ Fred Frederikse is a self-directed student of geography and traveller, and co-chair of the Whanganui Musicians Club. His millisphere columns can be accessed at millisphere.blogtown.co.nz
