Instantly one of the most remarkable transitions in history occurred: the youth grew up almost in a day and at once began to exert unsuspected reserves of power and authority.

In despair a number of the chief Florentines made an effort to depose him, and a battle was fought at Montemurlo, a few miles from Florence, between Cosimo's troops, fortified by some French allies, and the insurgents.

That was in 1537; the victory fell to Cosimo; and his long and remarkable reign began with the imprisonment and execution of the chief rebels.
Although Cosimo made so bloody a beginning he was the first imaginative and thoughtful administrator that Florence had had since Lorenzo the Magnificent.

He set himself grimly to build upon the ruins which the past forty and more years had produced; and by the end of his reign he had worked wonders.

As first he lived in the Medici palace, but after marrying a wealthy wife, Eleanora of Toledo, he transferred his home to the Signoria, now called the Palazzo Vecchio, as a safer spot, and established a bodyguard of Swiss lancers in Orcagna's loggia, close by.