LONDON (Reuters) -- The election of Muhammad Mursi as the first democratically chosen Islamist president in the Arab world represents an historic achievement for the Muslim Brotherhood, the most influential religiously-based movement in the Arab world.

After decades of persecution and incarceration, what is unfolding today clearly shows the weight and influence of the Muslim Brothers, most of whom are centrist and modernist and accept democratic values, in shaping the political future of their society.

But beyond the historical significance of Mursi's election as president of Egypt, he will most likely be a transitional president -- neither the commander-in-chief nor the executive. His presidency will be the weakest since the establishment of the Egyptian republic after the army toppled the old regime in 1952. He cannot declare war without the consent of the ruling generals and cannot veto the newly-established Defense Council in which the military has a voting majority.

The ruling generals will call the shots from behind and will be the driver. In fact, it is doubtful if the ruling generals would have allowed Mursi to assume office without a deal being reached with his Islamist movement.

A compromise sees the army in charge of national security and foreign policy, while Mursi and the civilian leadership tackle the broken economy and fragile institutions.

With minor exceptions, there will be no qualitative shift in Egyptian regional and international politics. The ruling generals will go to great lengths to maintain a monopoly on pivotal decisions in national security. They will fight tooth and nail to prevent Mursi from shifting Egypt's regional and international alignments.

After the celebrations subside, the fierce political struggle will resume with a vengeance. In the coming year there are big battles to be waged over the writing of the new constitution, the parliamentary elections, relations between the military and the civilian leadership, and the nature of the political system - presidential or parliamentary. Mursi will be pressed between a rock - the ruling generals - and a hard place - the rising expectations of the Egyptian people. The political fortunes of the Muslim Brothers will ebb and flow depending on how they manage to navigate this fierce power struggle.

Ideology takes a back seat to interest

Notwithstanding the supposed relationship between the military and the Muslim Brotherhood, the triumph of Mursi was not surprising. In the last four decades, the Muslim Brothers, along with other Islamists in neighboring countries, skilfully positioned themselves as the alternative to the failed secular authoritarian order. They invested considerable capital in building social networks on the national and local levels, including non-government professional civil society associations, welfare organizations, and family ties.

There is little to fear from the rise of Islamists to power. For more than four decades, the Muslim Brothers labored to enter politics and gain legal status. They learned the art of compromise and pragmatism through hardship and persecution. On balance, ideology takes a back seat to the interests and political well-being of their movements. More than ever, their message targets specific constituencies and interest groups -- a sign of an ideological shift and maturity. Arab Islamists are traveling a similar path as did the Christian fundamentalists and later the Christian Democrats and Euro-communists in Western Europe who in the 20th century subordinated ideology to interests and political constituencies.

Three big points. Firstly, the Muslim Brothers are slowly moving away from their traditional agenda of establishing an authoritarian Islamic state and imposing Islamic law to a new focus that is centered on creating a "civil Islam" that permeates society and accepts political pluralism.

Secondly, Islamist parties, including the Brotherhood's Freedom and Justice, are increasingly becoming "service" parties, an acknowledgment that political legitimacy and the likelihood of re-election rests on the ability to deliver jobs, economic growth, and to demonstrate transparency. The example of Turkey, especially its economic success, has had a major impact on Arab Islamists who believe that Islam and capitalism are mutually reinforcing and compatible. The Arab Islamists have, in other words, understood that "It is the economy stupid!"

Finally, despite their rhetoric, the Muslim Brothers continue to mellow in the arena of foreign policy and have shown a willingness to work with Western powers when their interests converge. This includes their posture toward Israel. The Islamists' commitment to Palestine, rooted in popular pressure from their constituencies, will mean that, while they will not renege on existing peace treaties, their relationship with Israel will remain frozen in the absence of a just solution to the Israel-Palestine conflict that is endorsed by Hamas.

Regardless whether the Muslim Brothers are allowed to carry out their agenda, one point must be made clear. The revolutionary moment is still unfolding with unpredictable turns and twists. Although in the short term the military is the driver, it is fighting a losing battle in the long term. The reasons lie in a mobilized public opinion that, time and again, has forced the ruling generals to retreat, as well as a relative consensus among differing political groups that the military must relinquish power to an elected civilian authority. In this sense, the triumph of Mursi is the first nail in the coffin of military's rule in Egypt.

Fawaz A. Gerges is Professor of International Relations and Director of the Middle East Centre at the London School of Economics. His new book, "Obama and the Middle East: The End of America's Moment?" came out this month.