The most interesting observation in John Mearsheimer\'s latest book is that national leaders rarely lie to each other. This depends on the notion of lying being restricted to untruthful statements, excluding subtler forms of misdirection. Politicians quite often lie to their own people in this narrower sense. One might expect them to do it to each other, too: the very definition of an ambassador, according to the 17th-century diplomat Sir Henry Wotton, is \"an honest man sent abroad to lie for the good of his country\". Nevertheless, if one accepts that deceptions are hard to maintain in the long run, it is striking how many cases of domestic lying have come to light and how few cross-border whoppers ever do. Mearsheimer, a political-science professor at the University of Chicago, asked his colleagues to wrack their brains for cases of the latter: they could only come up with a handful of clear-cut examples. Those, intriguingly, are often between allies. When Greece was trying to enter the euro it was running a budget deficit far larger than the EU\'s three per cent maximum, so it just cooked the books. Israel lied to the US to conceal its own secret nuclear weapons programme. The principle may seem strange, until one considers that it\'s hard to con someone who doesn\'t trust you in the first place. In addition, the costs of getting found out tend to be lower when one\'s victim wants to stay on friendly terms. These observations may also go some way towards explaining the prevalence of lies intended for home consumption: people by and large want to trust their leaders. And so we see that, in the run-up to the Iraq war, Saddam Hussein was actually telling the US the truth when he said he didn\'t have any weapons of mass destruction. Meanwhile, the US government told four big lies to its own citizens. It said it had cast-iron evidence that Saddam had WMDs, that it knew he was working with Osama bin Laden, that Saddam was connected with the 9/11 attacks in some way, and that he could avoid a fight if he co-operated more. In fact, the US had secretly committed to war quite early in its negotiations with Iraq. The title Why Leaders Lie is perhaps misleading, if one hears a moral or existential note to its \"why\". Mearsheimer pays almost no attention to diagnostic questions. His book might better be called The Strategic Imperatives Under Which Governments Are Likely to Lie, and he itemises those circumstances from a position of worldly impartiality. Both at home and abroad, it goes without saying that wars are a major cause of state subterfuge. Mearsheimer quotes a remark by the British politician Arthur Ponsonby: \"there must have been more deliberate lying in the world between 1914 and 1918 than at any other period of the world\'s history\". States must lie to one another to protect their strategic interests, and such fictions tend to be sold on the home front too, if only to preserve secrecy. At the same time wars also involve a lot of lying specifically intended for the citizenry. Threats are exaggerated to make action seem more urgent, as in the Iraq war, or, more nobly, Roosevelt\'s stories about the German attack on the USS Greer in 1941, by means of which he hoped to galvanise the US into joining the Second World War. It is common to cover up mistakes in order to maintain morale, and more generally to bowdlerise one\'s national history, to make it seem cleaner than it really is. After 1945 German leaders attempted to portray the Holocaust as an SS-only affair, so as to exculpate the bulk of the Wehrmacht, despite the fact that much of the German military machine was actively involved in the extermination of Jews and other despised groups.