Ian Hall is a Senior Fellow at the Australian National University.

Every student of international relations has, at some point, been required to read Kenneth Waltz, who died on 13 May aged 88. He was the preeminent international theorist of the post-war period, a thinker who produced not just one iconic work, but three: Man, the State and War (1959), Theory of International Politics (1979) and The Spread of Nuclear Weapons: More May Be Better (1981).

Waltz began his career as a political rather than an international theorist. Man, the State and War, which started life as a PhD thesis, plundered the history of political thought for rare snippets of thinking about relations between states that might be put to good use. From these snippets Waltz assembled three 'images' of international relations: one that explained the behaviour of states in terms of the drives, faults and possibilities inherent in human nature and individual human leaders; one that explained it in terms of the character of the domestic politics of states; and one that explained it in terms of the structure of the international system.

Waltz dismissed the first and second images as unhelpful to theorists. This was a bold, possibly even career-wrecking move for a young scholar, for it cast aside almost all the work done in the field of international relations for more than thirty years by many eminent professors.

Both classical realism and liberalism, the two theoretical schools that had vied for influence since the 1930s, relied on the first or second images to explain world politics. For classical realists like Hans Morgenthau, war and competition in international relations arose from the hunger for power in 'men' (as they put it); for liberals like Alfred E Zimmern, war was rife because tyrants were unrestrained by law within and outside their states.

Waltz's dismissal didn't quite consign these theories to the dustbin of history, but he did damage to both, demanding their reconsideration.

In Theory of International Politics, Waltz turned from destruction to construction and tried to do two things. First, he argued that the behaviour of states was best explained by the third 'image'. The structure of the international system, he argued, conditioned and thus explained state behaviour. Because that structure is anarchical, lacking a sovereign to make and enforce the law, cooperation between states was difficult and competition endemic.

Because the states that make up the system are equal in formal terms, but very unequal in their capabilities, being rich and strong or poor and weak or somewhere in between, they do however sometimes cooperate to balance against particularly powerful states that threaten others. Because of anarchy, states seek to maximise their power to provide for their own security; because of hierarchy, they sometime ally to maintain the balance of power.

Second, Waltz tried to show how international relations theorists ought to do their job. He argued – much to the continuing disgust of many political scientists – that theories are not built by painstakingly testing hypothesis after hypothesis. Even if a hypothesis is proved true (or at least, statistically most likely), the result, Waltz insisted, was a description, not an explanation. Theory must go further, and explain why regularities or correlations occur. 'To form a theory', he wrote, 'requires envisioning a pattern where none is visible to the naked eye.'

These arguments have provoked almost as much debate over the past thirty years as Waltz's 'structural realist' explanation of international relations. Equally controversial was his claim, made in a famous monograph published by the International Institute for Strategic Studies, that more nuclear weapons 'may be better'.

This argument captured the provocative, sometime contrary, always rigorous essence of Waltz. If the advent of nuclear weapons had led to more than three decades of peace between two implacably-opposed superpowers, Waltz asked, why not let other states acquire them? Perpetual peace could be achieved not by federation, as Kant thought, but by proliferation. A few bombs each would be enough, Waltz thought, to deter even the most fervently ideological, reckless or feckless leader from attacking their neighbours.

It was this logic that drove him last year to champion – in the august pages of Foreign Affairs, no less – an Iranian bomb.

There is much in Waltz's work to disagree with, and many have dissented from his theories. But there is no questioning the power of his thinking or the sheer weight of his contribution to the field, which was truly extraordinary.