Many of today's global problems are hangovers from bad, ungenerous decisions at the end of previous conflicts, writes Jeffrey Sachs.

This has been a year of great geopolitical anniversaries. We are at the 100th anniversary of the start of World War One, an event that more than any other shaped world history during the past century. We are at the 25th anniversary of the fall of the Berlin Wall, the opening chapter of the demise of the Soviet empire and the end of the Cold War. Yet we know that painfully we observe something far more than a mere remembrance.

As William Faulkner remarked, "The past is never dead. It's not even past." WW1 and the fall of the Wall continue to shape our most urgent realities today. The wars in Syria and Iraq are the legacy of the closure of WW1, and dramatic events in Ukraine are unfolding in the long shadow of 1989.

1914 and 1989 are "hinge moments", decisive points of history on which subsequent events turn. How nations both great and small behave at such hinge moments determine the future course of war and peace.

I participated directly and personally in the events of 1989, and saw this lesson in play - positively in the case of Poland and negatively in the case of Russia. And I can tell you that as I carried out my own tasks as an economic adviser during 1989-92, I kept a constant and always worried gaze on 1914. I carry that same sense of worry today.

In 1919, at the end of WW1, the great British economist John Maynard Keynes taught us invaluable and lasting lessons about such hinge moments, how decisions of victors impact the economies of the vanquished, and how missteps by the powerful can set the course of future wars.

With uncanny insight, prescience, and literary flair, Keynes's 1919 The Economic Consequences of the Peace predicted that the cynicism and shortsightedness at the core of the Versailles Treaty, especially the imposition of punitive war reparations on Germany, and the lack of solutions to the roiling financial crises of the debtor countries, would condemn the European economies to continuing crisis, and would in fact invite the rise of another vengeful tyrant in the coming generation.

Keynes's cri de coeur is one of those remarkable outpourings of genius that speaks across generations. That book and its lessons proved to be a formative guide for me in my own career as policy adviser and analyst.

As a newly minted economist some 30 years ago, I suddenly found myself charged with helping a small and largely forgotten country, Bolivia, to find a way out of its own unmitigated economic disaster. Keynes's writings helped me to understand that Bolivia's financial crisis should be viewed in social and political terms, and that Bolivia's creditor, the US, had a shared responsibility of resolving Bolivia's financial anguish.

My experience in Bolivia in 1985-86 soon brought me to Poland in the spring of 1989, at a dual invitation of Poland's final communist government and the Solidarity trade union movement that strongly opposed it. Poland, like Bolivia, was financially bankrupt. And Europe in 1989, like Europe in 1919, was at a great hinge-moment of history.

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