How did France became an empire?

Frequently. Apart from the early empies such as that of Charlemagne, which usually disintegrated on the death of their founder, the Empires most frequently referred to in France are the Napoleonic ones. The FIRST EMPIRE, then, was that of Napoleon I, and it fell twice; in 1814 when Napoleon abdicated and was exiled to Elba, and in 1815 when, having returned, he was defeated at Waterloo and exiled to St. Helena. In both cases, the result was the restoration of the Bourbon monarchy under Louis XVIII. The SECOND EMPIRE was founded in 1852 when the Prsident of the Second Republic, Louis-Napoleon Bonaparte (Great-Nephew of Napoleon I) was proclaimed Emperor as Napoleon III (Napoleon's heir, the King of Rome, who would have been Emperor on his father's death in 1821, died young and never attained a throne). This Second Empire fell on the defeat of France in the Franco-Prussian War of 1870, and was replaced by the Third Republic.