Egypt's pledge to help defend Saudi Arabia against Houthi attacks from Yemen has rallied the state and its allies, but it has also drawn calls for caution amid an escalating situation in the Middle East due to the impact of the Iran war.
From the steps required in parliament to the arguments put forward by different camps in Egypt, the nature of any military support remains uncertain.
Here is a breakdown of the practical steps involved and the debate unfolding in the country with the Arab world's largest population and largest army.
Vote and formalities
Parliament is expected to pass the request quickly. The constitution requires approval by two-thirds of the 596-member House, or 398 votes, before troops can be sent on a combat mission abroad. The four main pro-government parties alone won 403 of the 568 elected seats in last year's election.
Approval would give President Abdel Fattah El Sisi, as supreme commander of the armed forces, the legal authority to deploy troops, but it would not send them automatically. Egypt will act only if Riyadh asks, and no formal Saudi request has been announced.
The National Defence Council, chaired by Mr El Sisi, met at Al Ittihadiya Palace on Sunday. It declared Egypt's "full solidarity" with the kingdom and pledged "all necessary and possible forms of support" to defend its security, its territory and the Islamic holy sites.
The council met a day after Mr El Sisi told a graduation ceremony at the Egyptian Military Academy that it was "inconceivable" Egypt would allow Saudi Arabia's security to be harmed. He asked parliament to authorise action "if we are asked".
Neither statement said what form the support would take, how many troops might be sent or for how long. That silence has shaped much of the reaction since.
The pledge follows near-daily Houthi attacks on Saudi Arabia since early October. The deadliest attack was at Riyadh's King Khalid International Airport, which killed and injured hundreds. Among them was Azza Ramzy, a 28-year-old Egyptian flight attendant from Beheira province.
Loyalist support
The vote has been pushed from Sunday to Tuesday amid reports of heated debate in the house.
About 300 MPs asked to speak in the closed sessions, according to local media. Some MPs have also pressed for details on the mission's nature and scope before they cast their vote.
Three MPs said they would abstain in the vote. However, Mostafa Bakry, an MP and TV host close to the state, played down any rift. "There was an overwhelming majority," he said. "No MP has rejected the authorisation.
Within ruling circles, support has been swift and near unanimous. Pro-government Mostaqbal Watan, the largest party in parliament, declared its backing as MPs convened on Sunday, followed by Homat Al Watan, the National Front and the People's Republican Party. The Salafist Nour Party also backed the request. El Sayed El Badawi, leader of the Wafd, Egypt's oldest liberal party, said Saudi Arabia's security was "an inseparable part" of Egypt's own.
Supporters point to the Red Sea, where Houthi attacks on shipping have already cost the Suez Canal heavily in lost revenue. They also point to gratitude for Saudi's continued financial support of Egypt. Central bank data show a $5.3 billion Saudi deposit, part of $10.3 billion Riyadh holds at Egypt's central bank, maturing this month, with Cairo facing a potentially costly repayment unless Riyadh agrees to roll it over.
Pro-government figures have cast the move as purely defensive. TV hosts told viewers it concerned protecting Saudi Arabia, not declaring war or going into another country.
Questions and more
Between the loyalists and others who question the move, sits a camp that backs Saudi Arabia but wants clarifications on any mission.
The Egyptian Social Democratic Party told its MPs to abstain. It said its solidarity with Riyadh was "steadfast", but Egypt's national security "requires clarity of objectives", and military participation must not become "an open-ended commitment". The party also cited lost canal revenue and the strain on the state budget.
Ayman Nour, the exiled leader of the liberal Ghad Party, wrote to Speaker Hisham Badawi before Sunday's session. Defending Saudi Arabia was "an Arab duty", he said, but "authorisation is not a blank cheque". He listed 10 safeguards, including a fixed duration, a defined geographical scope, and regular reports to parliament. He cited Egypt's intervention in Yemen in the 1960s, a war many Egyptians still call "Egypt's Vietnam", as a lesson.
Hamdeen Sabahi, a former presidential candidate, went further. Egypt should be "a sponsor of reconciliation", he wrote on Facebook, and should "not send its army to defend one brother, but to establish peace between the two brothers".
Al Azhar, the country's highest seat of Sunni learning, called for "an immediate and comprehensive halt" to fighting in the region and proposed an urgent Arab and Islamic mediation committee. Others reject a combat role altogether. On the far left, the Revolutionary Socialist Current issued a statement headlined "No to war". Business leaders have largely stayed out of the debate.