The men who wrote the Constitution had just finished overthrowing a king, and their great fear was building a new one by accident. So they refused to put power in one place. They split it three ways: Congress makes the laws, the president carries them out, the courts judge disputes under them. That split is called the separation of powers. Nobody holds the whole thing.
Then they set the parts against each other — the checks and balances. Congress passes a bill; the president can veto it, refuse to sign it into law; Congress can override the veto with a two-thirds vote of both houses. The president commands the armed forces, but only Congress can declare war — and Congress holds the money: not a dollar moves without it. The president appoints judges and top officials; the Senate must confirm them. And Congress can impeach a president or a judge — bring formal charges of serious wrongdoing — after which the Senate tries the case, with a two-thirds vote needed to remove. Three presidents have been impeached, one of them twice. None has been removed.
The courts got the quietest check and the largest. In 1803, in a case called Marbury v. Madison, the Supreme Court claimed the power of judicial review: the power to strike down a law that violates the Constitution — in effect, the power to say what the Constitution means. The document nowhere states that power in so many words; the Court reasoned its way to it, and the other branches accepted it. Ever since, the last word in many of the country’s biggest fights has belonged to nine unelected judges, which is why battles over who sits on that Court are among the fiercest in American politics.
The design got its hardest test in the 1970s. In the Watergate scandal, men working for the president’s re-election broke into the opposition party’s offices, and the president joined the cover-up. Then the machine engaged: reporters dug, Congress investigated on live television, and the courts ordered the president to surrender his secret tapes. When the Supreme Court ruled against him unanimously, he obeyed — and resigned in 1974, ahead of certain impeachment. Each branch did its work. The machine held.
One honest note to carry with the diagram: none of these checks operates by itself. They are used by people, or left unused. The founders designed the branches to be jealous of each other; they did not plan for political parties, which can make a Congress protect a president of its own party instead of checking him — a pull felt in every era and by every party. Whether the machine holds next time depends, as it did last time, on the people inside it, and on the voters watching them.