Part of Morsi's platform will be the institution of some form of sharia law as he promised during his campaign. REUTERS

The long-anticipated showdown between Egypt's military and its new president -- the Muslim Brotherhood's Muhammad Morsi -- has commenced.

Following Morsi's surprise decree on Sunday that the previously dissolved parliament would go back to work, Egypt's Supreme Constitutional Court invalidated the order a day later, declaring that its previous ruling was final and not subject to appeal.

Egypt's lower house of parliament was disbanded prior to Morsi's victory late last month, when the high court ruled that a third of the members of the People's Assembly, many of them from the Brotherhood's Freedom and Justice Party, had unlawfully run for seats reserved for independents. The ruling was upheld by the Supreme Council of the Armed Forces, or SCAF, which has been in control of Egypt since the end of last year's revolution that toppled former President Hosni Mubarak.

But with the presidential decree, Morsi seemed to be testing the limits of his 10-day-old presidency and challenging both the SCAF and the courts. By Monday, regional papers such as Kuwait's Al-Watan and Egypt's Al-Tahrir had already declared a victory for Morsi over the military.

Additionally, Saad al-Katatni, the Speaker of the People's Assembly, promised that parliament would resume at noon on Tuesday. Earlier, the army guards that have surrounded the parliamentary building in Cairo for weeks made way for some MPs entering the premises.

But, the constitutional court's sudden ruling on Monday afternoon could have nullified Morsi's progress and brought the president his first defeat.

With his decree on Sunday, Morsi may have been trying to pressure the SCAF into quickly approving a new constitution. When the lower house was disbanded, a constitutional draft committee was scrapped along with it. Days later, on the eve of the presidential election, the military gave itself sweeping powers, including the final endorsement of Egypt's future constitution.

But Morsi announced that the elections to replace invalidated parliamentary members would take place 60 days after the constitution had been approved. This effectively meant that the SCAF would have to swiftly move on a constitution if it wanted to replace the Muslim Brotherhood-dominated parliament.

Despite making nice with Morsi at the armed forces graduation ceremony on Monday, SCAF leader Field Marshal Hussein Tantawi is not taking the presidential decree lightly. The military held an emergency meeting behind closed doors on Sunday, the result of which is unclear, and was expected to hold a follow-up session on Monday.

According to local sources who spoke to Al Arabiya, the Supreme Constitutional Court's ruling was a surprise to everyone, including the military council.