Muhammad Ali of Egypt with his son, Ibrahim
As the year 1840 progressed, the armies of Muhammad Ali Pasha of Egypt stood firm at the great Euphrates river. For the Albanian military man, the circumstances were a dream come true. His son had occupied the long coveted land of the Levant, all the while thwarting the Ottomans at Nizib. What's more, he had control of the Ottoman navy, one which would have been a thorn in the side of the ambitious leader. If the conflict was over here, he thought, he would have carved out a nice state for himself and his successors. However, the leader was a man of grandiosity, and his son/general, Ibrahim, had greater aims: The conquest of Constantinople. Rather than carving out a state from the "sick man of Europe", Ibrahim envisioned his family as the hegemon of the vast, albeit ailing, realm. Perhaps his family could restore the nation to its former glory, when it was Europe's scourge rather than its pawn.
Ibrahim's plan would have to rely on one thing: The support of more governors, especially from the European eyalets. Many were impressed with the economic and military development of Egypt, and if Constantinople were to fall to Muhammad Ali's armies, he was confident their support would fall to them in no time.
So it was set. The armies, 40,000-45,000 strong with ample reinforcements on the way, would make the trek towards Constantinople, once the city of the world's desire.
[1]
Ironically, the European powers would decide the fate of this nation. Much of Europe was watching with a concerned and panicked eye, each with its own desires and goals. At the time, the powers of the continent were in a string of alliances and uncertainties. Many states seeked a stable, albeit weak, Ottoman state to keep stability in the region. For the United Kingdom, this was a must. One of her greatest fears was Russian control of the Bosphorus straits, dragging the Balkans into its sphere of influence.
[2] The UK found agreement with this in Austria, and thus both moved to oppose Ottoman collapse.
Russia, of course, would vastly enjoy more influence in the region. A destabilized Ottoman empire, however, could leave a lot of variables at play. Nothing would be worse than another nation filling the Ottomans void. However, this seemed less and less likely. Russian diplomats and leaders would have to figure out how much they could get away with without antagonizing British or Austrian diplomats.
And then, there was France. France, after recently conquering the vast expanses of Algeria, desired to increase its outreach in Northern Africa, of which pitted it against the Ottomans. Also coveted was expanding its trade influence into the Levant and the greater Middle East. However, Russian control of the Bosphorus was just as undesirable to French interests as the rest of the European powers. Besides, the Napoleonic wars were still fresh in the memories of the French government. So it appeared that the best case scenario would be to weaken the Ottoman Empire without allowing too much Russian interference or provoking the British. With this in mind, the French set out to negotiate peace. While France rushed to insure a favorable peace for them and the Egyptians, the British and Austrians would begin to work out a solution of their own
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[1] - Here is our first major POD. Unlike OTL, Muhammad Ali adheres to the will of his son, and begins to lead a campaign to Constantinople. The more hasty approach here causes the European powers to panic, act faster, and ultimately, concede more.
[2] - This was a concern of the UK that would lead to the Crimean War in OTL. In this situation, the relationship between the two states sour just a little earlier.