The Law of Fratricide has a complex history and the exact way in which it manifested itself changed over time. For the uninitiated, this is the tradition established by Mehmed II (r. 1451-81) that a sultan should kill all of his brothers upon becoming the ruler in order to prevent them from fomenting sedition and attempting to take the throne themselves. It was intended to cause stability, but essentially guaranteed a civil war upon each succession to the throne, and in some cases even before the ruler's death. However, this was deemed better than allowing the brothers to live, potentially able to launch a rebellion at any moment, as that would put the state in a permanent condition of danger. Better to get the fighting out of the way in the beginning. It also ensured that the Ottoman state could not be divided, with different sons claiming shares of the empire as inheritance.

Traditionally Ottoman princes were sent as governors to the provinces upon reaching the age of maturity in order to acquire practical experience with running the empire. This was where they built up their power base from which to launch their bid for the empire. During the reign of Süleyman the Magnificent (r. 1520-1566) the empire witnessed particularly brutal warfare between his sons during the last decade and a half of his reign. Whether a direct result of this or not, the next two generations of Ottoman rulers began practicing what the historian Leslie Peirce calls "temporary reproductive monogamy," such that they bore only one son with one concubine before coming to the throne, after which they were free to take up other concubines and have more children. This had critical consequences, for when Selim II died eight years into his reign (1574), his son Murad III was the only prince elderly enough to possess a provincial governorship - Selim's five other sons were still children in the palace. Thus they were easy targets for Murad to execute. Murad likewise had only one son at the time, the future Mehmed III. When Mehmed took the throne he had his young brothers executed yet again, but this time there were 19 of them - Murad had been prolific with his concubines. This horrified the population of Istanbul. Rather than princes proving their worth on far-away battlefields as in the past, now the leading prince was simply massacring innocent and helpless children in the palace, and the population was witness to their many coffins being paraded out of the palace - the mausoleum in which they are buried is still open to the public in Istanbul today, just outside Hagia Sophia.

This is one of the reasons why, when the next sultan, Ahmed I (r. 1603-1617) came to the throne, he decided not to have his brother Mustafa killed. Ahmed I was only 13 years old, not quite at the age necessary for traveling to a provincial governorship, and in fact it was from this point that the practice came permanently to an end. It's unclear why Mustafa I was left alive, but the most likely explanation is that he was the only other living male of the Ottoman dynasty. Before, all princes who took the throne already had children of their own when they did so, thus killing their brothers did not leave the dynasty in danger. Ahmed I was still too young to have children, and it was probably considered too great of a risk to base the entire future of the dynasty on his untested ability to procreate. When Ahmed I died in 1617 it was his brother Mustafa rather than his eldest son Osman who became sultan, marking a shift in Ottoman succession practice to seniority. Although Osman was 14, older than his father had been when he took the throne, it seems that Ottoman attitudes toward succession had definitively changed in the preceding 40 years.

Now, fratricide did still occur in the time after Ahmed I, but it was not an officially sanctioned and accepted action to be taken immediately upon succession. Henceforth, sultans were obliged to request the right to execute their brothers from the empire's chief religio-judicial figure, the Şeyhülislâm, who did not always grant this permission. This is part of what Baki Tezcan has identified as the introduction of limited government into the Ottoman Empire. Whereas the sultans of the sixteenth century were absolute rulers, with no living brothers who could possibly threaten them, and only at times endangered by their sons, the Ottoman sultans of the seventeenth century were beholden to their political supporters in Istanbul because they could potentially be overthrown at any time and replaced by another member of the dynasty living in the palace. The dynasty was again endangered in one instance, when the unusually powerful and autocratic sultan Murad IV (r. 1623-40) died with no living heirs, placing the entire future of the empire on the shoulders of his brother Ibrahim I (r. 1640-8), who was successfully able to father the next generation.

As with most such material on Ottoman dynastic reproduction, this post is largely based on Leslie Peirce's 1993 The Imperial Harem: Women and Sovereignty in the Ottoman Empire. Tezcan's book is The Second Ottoman Empire: Political and Social Transformation in the Early Modern World (2010), certainly one of the most important books on the Ottomans of the past decade, exploring the changing nature of the Ottoman dynasty's authority in the late sixteenth and early seventeenth centuries.