Why do so many countries split power into three branches?

Many countries divide government into a legislature that makes laws, an executive that carries them out and courts that judge, because concentrating all three in the same hands makes tyranny easy. The French thinker Montesquieu put it bluntly in his 1748 book The Spirit of Law: if the same body both makes and enforces the laws, it can pass tyrannical laws and then carry them out tyrannically. If judges also make the laws, power over people's lives becomes arbitrary, because the judge is the legislator. And if one person or group held all three powers, he wrote, all would be lost.

Montesquieu was a baron who held office for twelve years in the Bordeaux Parlement, and he built his idea by looking abroad. He studied the Roman Republic and spent more than a year in England, where he saw power shared among the monarch, Parliament and the courts. His book was banned by the Catholic Church in 1751, but it found eager readers across the Atlantic. American colonists quoted him more than any authority except the Bible, and James Madison called him "the oracle who is always consulted and cited".

The US Constitution of 1787 turned the idea into machinery. It is not enough to separate the branches; each needs ways to defend itself from the others, through checks and balances such as vetoes. Madison's reasoning was very human: if people were angels, no government would be needed, but since people govern people, government must be able to control itself.

The design has a cost. When the branches disagree, vetoes can produce gridlock. Many democracies also blend powers more than the textbook model suggests.