The French Revolution of 1789 overthrew an ancient monarchy and replaced it with a republic. Unlike the American republic, which prospered, the French republic was beset by enemies. The republic militarized; its armies crossed Europe. In 1799, it was toppled by Napoléon Bonaparte, making it the first modern republic to be replaced by an authoritarian regime.
The Revolution also set loose forces felt far beyond Europe. It prompted slave revolt in the Caribbean, weakened the hold of Europe on the Americas, and shook the status quo in North Africa and the Middle East. Feminism, secularism, political terror, conservativism, and human rights were founded or sharply inflected by the Revolution. If you want to understand contemporary politics, this is the place to begin. In almost every sense that matters, political modernity begins in 1789.