In 1791, the National Assembly ratified a new constitution, which was somewhat similar to the u.s. Constitution ratified just two years before, except that instead of a president, the king held on to the executive power. In other words, it was a constitutional monarchy, rather than a constitutional democracy. Those who wanted to abolish the monarchy felt cheated; those who wanted to retain the feudal structure felt betrayed.
Remember how most of the royalty in Europe intermarried? Well, it just so happened that Marie Antoinette, who was the wife of the increasingly nervous Louis XVI, was also the sister of the Emperor of Austria. The Austrians and the Prussians invaded France to restore the monarchy, but the French revolutionaries were able to keep them back. Continuing unrest led French leaders to call for a meeting to draw up a new constitution. Under the new constitution, the Convention became the new ruling body, and it quickly abolished the monarchy and proclaimed France a republic. Led by radicals known as the Jacobins, the Convention imprisoned the royal family and, in 1793, beheaded the king for treason.