Egyptian military and security officials told the Associated Press on Thursday that Egypt, Saudi Arabia and other Arab states plan to invade Yemen once airstrikes have sufficiently weakened the Houthi rebels and forces loyal to the country’s former strongman, Ali Abdullah Saleh.

They said the assault will be by ground from Saudi Arabia and by landings on Yemen’s Red and Arabian Sea coasts.

The officials did not say when the operation would start nor did they specify the number of troops likely to be involved.

Egypt’s presidency said in a statement on Thursday that its air and naval forces were already participating in the Saudi-led campaign that began on Wednesday.

Sameh Shoukri, the foreign minister, said Egypt was “prepared for participation with naval, air and ground forces if necessary.” Details could be unveiled at an Arab summit in the Egyptian resort of Sharm el-Sheikh this weekend.

“The Egyptians’ decision to return to Yemen, which is essentially their Vietnam, is extremely significant,” argues The American Interest‘s Walter Russell Mead.

It reflects both enormous fear on the part of the Sunni powers and the strength of the Saudi-led alliance.

Egypt deployed tens of thousands of troops to Yemen during the 1962–1970 North Yemen Civil War to support republicans against Saudi-backed royalists. The republican side ultimately prevailed but at huge cost to Egypt’s treasury and morale.

Like Saudi Arabia, Egypt’s army regime worries that the United States, its ally, will acquiesce in recent Iranian strategic gains in Iraq — where Tehran supports the Baghdad government’s fight against Islamic State jihadists — under an agreement that should stop it from obtaining nuclear weapons.

Egypt also shares Saudi Arabia’s concerns about the spread of political Islam in the Middle East, an often violent ideology that challenges the legitimacy of both its own secular, military-led government and the Saudi monarchy. Egyptian jets attacked Islamist militants in Libya last month where the United Arab Emirates earlier intervened on the side of the internationally-backed government in Tripoli.

Some one hundred Saudi jets started bombing in Yemen Wednesday night, targeting the presidential palace as well as police and military headquarters in Sana’a, the capital.

The Saudi-owned news channel Al Arabiya reported that 150,000 of the kingdom’s troops were involved in the operation. Aircraft from Bahrain, Egypt, Jordan, Kuwait, Morocco, Qatar, Sudan and the United Arab Emirates were expected to join the fighting.

The Arab states said they were responding to the internationally-recognized government’s request for intervention. President Abd Rabbuh Mansur al-Hadi and his ministers were forced to seek refuge in the port city of Aden, Yemen’s second largest, after Houthi rebels from the north dissolved parliament and took over Sana’a in February. They have advanced in the direction of Aden since.

Sudan’s participation in the coalition in surprising given past reports that it helped Iran arm the Houthi rebels.

David Andrew Weinberg, a senior fellow at the Foundation for Defense of Democracies, writes for Al Arabiya that the turnabout came after meetings between Saudi king Salman and Sudan’s president, Omar Bashir, on Wednesday. Sudan would contribute only three fighter jets to the effort.

Sudan’s main contribution sought by Riyadh was to steer clear of assisting Iranian naval activities in the Red Sea.

While Saudi Arabia and the other Sunni Arab states support Hadi, Iran has backed the Houthis who share its Shia faith. A delegation of Houthi leaders traveled to Tehran for talks earlier this month. Iranian medical supplies arrived in Yemen a day after the two regimes signed an aviation agreement.

Iran and Saudi Arabia are locked in a struggle for hegemony in the Middle East and also back opposing sides in the Syrian Civil War. Iran supports the regime of President Bashar al-Assad while Saudi Arabia has armed and financed the largely Sunni uprising against him.

Hadi’s predecessor, Saleh, has joined the war against government loyalists although it is unclear to what extent his supporters coordinate with the Houthis whom they previously opposed.

Saleh, who presided over Yemen’s unification in 1990 and a civil war that started in 1994 when the south tried to secede, was forced out of office by his Arab Gulf neighbors in 2012 amid fears of an “Arab Spring”-style uprising in the impoverished Arab country.