According to Mearsheimer, nations base their policies on a relentless drive for greater power and to check the power of potentially rival nations. Human rights, ideology, and peace considerations are secondary at best. Nor does a country's political system make much difference; no matter how free and democratic a country may be at home, it will still behave in the same dog-eat-dog way abroad.
Perhaps one of the most interesting ways to look at Mearsheimer's thought is how neatly it dovetails with the most left-wing critiques of American foreign policy. The United States did not enter World War II to liberate the victims of Hitler's genocide; it did so to prevent the emergence of a single dominant European power. The Cold War was not waged to free the world from communism, but simply to contain a rival power. Similarly, Soviet policy during the Cold War was not based on a fanatical desire to tyrannize the globe, but simply to expand its own power. Democracy and authoritarianism both ultimately respect the one ideology of international affairs: nationalism.
But although he may be correct in the motivations behind state behavior, his discussion is deeply flawed by its near-exclusive concentration on military power. Economics is only important insofar as it can be used to finance and develop the military.
As such, Mearsheimer hardly touches on international trade or finance. This is deeply shortsighted, since in practice that is where a large part of the conduct of foreign policy lies. Nor would such a discussion weaken his theory; it would only strengthen it.
It explains why the United States never tried to form a colonial empire as Britain and France did; it was content, instead, to allow other regions to retain their independence and govern their own affairs as long as the U.S. had access to their resources and labor.
If economics is added, this almost perfectly explains U.S.-Canadian relations along the most leftist lines. According to a purely territorial application of Mearsheimer's theory, the US would have invaded and conquered Canada in the mid-19th century; it did not because it already has as much access to Canadian resources and markets as it needs. It also explains the situation today, where Canada is permitted to maintain political independence, but economically remains essentially an American protectorate.
Mearsheimer points out that the existence of nuclear weapons on both sides of an arms race does not prevent the race from continuing in conventional weapons. Nuclear weapons do not buy security, but increase insecurity. By his logic, states like Iraq and North Korea that seek to acquire nuclear capability are doing so in perfectly rational self-interest, giving the lie to fatuous "axis of evil" claims - revealed as the rhetoric they are to promote simple American interest in remaining the "offshore balancer" in each respective region.
More troubling, though, is his assertion at the end of the book that the U.S. should seek to slow or reverse China's economic growth. Taken literally, this seems to mean that it is apparently in the American interest to sabotage or undermine the Chinese economy, despite its growing importance as a market for American products. Implied by this is the spectre of renegade economic warfare, various schemes to undermine a rival's economy and thus its military capability. If that happened, the Chinese would undoubtedly try to retaliate in kind, and before long every great power would descend into al-Qaeda type small attacks and subversions. Not a pleasant prospect.
Perhaps what is most saddening about this book is that the reason for war is given simply as the desire by states to either become a hegemon or prevent someone else from doing so. Dusty, academic theories of the balance of power are the reason millions of people are killed in senseless and brutal fighting.
Another flaw in the book is it concentrates largely on Europe and northeast Asia, paying scant attention to the rest of the world. This flaw becomes most glaring when he portrays the bipolar Cold War as a peaceful period, ignoring the fact that it was peaceful only in Europe. People in El Salvador, Nicaragua, Chile, Angola, Mozambique, Vietnam, Indonesia, Ethiopia, Somalia, and many other countries that had oppressive dictatorships installed, or brutal insurgencies financed (or both) by superpower meddling would beg to differ.
What really happened is actually described in the book, albeit in a different context - buck-passing. The Soviets, rather than fighting the U.S. directly, armed their proxies in North Vietnam and elsewhere. The Americans, similarly, overthrew the government of Chile and funded armed rebellions in several countries against regimes believed to be friendly to the USSR. While the superpowers played their chess game, millions in the Third World paid with their lives.
This is a well-written and thought-provoking book, but cannot be relied upon for a complete overview of world affairs. More and diverse perspectives are needed.