Saudi Arabia and its Gulf region allies from the GCC launched military operations including air strikes in Yemen, and threatened ground troops against rebels in that country, inserting itself into its southern neighbour's civil war and potentially opening up a broader sectarian conflict in the Middle East.
The swift and sudden action involved 100 Saudi jets, 30 from the United Arab Emirates, 15 each from Kuwait and Bahrain, and 10 from Qatar, plus a handful from Jordan, Morocco and Sudan – along with naval help from Pakistan and Egypt, according to a Saudi adviser.
These countries are all predominantly Sunni Muslim, in contrast to the Shia Houthi rebels, who have taken over Yemen's capital of Sanaa and on Wednesday captured parts of its second largest city, Aden.
The Saudis consider the Houthis as proxies for the Shiite government of Iran, and fear another Shiite-dominated state in the region.
"What they do not want is an Iranian-run state on their southern border," CNN military analyst Lt Col Rick Francona said of the Saudis.
The United States did not send warplanes, though it has worked with Yemen's government, and backs the recently deposed but still battling President Abdu Rabu Mansour Hadi, to go after al Qaeda terrorists in the Arabian Peninsula.
A senior official in President Barack Obama's administration said "there will be no military intervention by the US".
But US Secretary of State John Kerry did tell foreign ministers from Saudi Arabia, the UAE, Qatar, Kuwait, Bahrain and Oman today that his country commends the military action and is supporting it through intelligence sharing, targeting assistance and logistical support.
Iran denounced the military intervention. Marzieh Afkham, a spokeswoman for Tehran's foreign ministry, said the operation will throw an already complicated situation into further turmoil and disrupt chances at a peaceful resolution to Yemen's months-long internal strife. It also won't help a region already facing terrorist threats from groups like ISIS in addition to al Qaeda.
"This is a dangerous action against international responsibilities to respect countries' national sovereignty," Afkham said, according to a report in Iran's state-run Islamic Republic News Agency.
And the move was met with defiance by the Houthis, the minority group that has emerged as the most powerful player in Yemen.
"This is a clear aggression and we will respond by a counteraggression," Ali Al Imad, a leading member of the group's political wing, Ansar Allah, told CNN Arabic. "The Saudi move will unite all the people of Yemen against the Saudis and the kingdom will pay the price."
In addition to airstrikes, the Saudi adviser said 150,000 troops could take part in an operation in Yemen.
Imad predicted that if the Saudis try to invade with ground forces, they'll fail.
"They probably will try to avoid that, but if it happened, then they will pay a very high price," he said.