US President Donald Trump's standoff with Democrats over a border wall with Mexico led to the longest-ever government shutdown in history, lasting nearly 35 days. As a second shutdown looms, we ask: how do government shutdowns work in America, why do they happen, and when was the first one? Here, historian Adam IP Smith explains how the separation of powers between the executive and Congress – also known as 'checks and balances' – is to blame, and warns that things may have to get a lot worse before they get better…

Like so much else, it’s all the fault of the founding fathers. They thought they were being terribly clever in hard-wiring a separation of powers into their constitution. The executive (i.e. the president) is separately elected from the legislature (the two houses of Congress), which makes the laws. And the judiciary (the highest branch of which is the Supreme Court) is appointed in such a way that its judges, once in office, are supposedly immune from interference from either of the other two branches.

The founders intended that, unlike a British monarch, an American president would be restrained by the Congress – and no form of restraint was more powerful than being able to control the money. Only Congress could raise revenue, and only Congress could authorise spending. Thus, the wings of a would-be tyrannical executive would be firmly clipped. Or so they thought.