After discussions in Paris, the European powers and the Ottoman Empire ratified Cuza's election, and the United Principalities officially became Romania in 1861. Almost immediately Cuza initiated a reform program. Encountering resistance from oligarchic boyars, the prince appealed to the masses and held a referendum that approved constitutional provisions giving him broad powers to implement his program. The government improved roads, founded the universities of Bucharest and Iasi, banned the use of Greek in churches and monasteries, and secularized monastic property. Cuza also signed an agrarian law that eliminated serfdom, tithes, and forced labor and allowed peasants to acquire land. Unfortunately, the new holdings were often too expensive for the peasants and too small to provide self-sufficiency; consequently the peasantry's lot deteriorated.

Cuza's reforms alienated both the boyars and Romania's mostly Greek clergy, and government corruption and the prince's own moral turpitude soon eroded his popularity. In 1865 an uprising broke out in Bucharest. Afterward, animosity toward the prince united the leaders of Romania's two political parties, the pro-German Conservatives, backed by the boyars and clergy, and the pro-French Liberals, who found support in the growing middle class and favored agrarian reform. On February 23, 1866, army officers loyal to the country's leading boyars awoke Cuza and his mistress, forced the prince to abdicate, and escorted him from the capital. The next morning street placards in Bucharest announced the prince's departure and rule by a regency pending the election of a foreign prince.