It is astonishing to consider the full arc of Henry Kissinger's influence on U.S. and world politics, spanning as it does close to six decades, 11 presidents and four generations' worth of dangerous events.

Indeed, it's hard even to remember a time when Kissingerian insights were not pulled out and debated whenever U.S. foreign policy faced a new round of soul searching.

Even in the distant past of the Eisenhower era, back before U.S. space flights, the Berlin Wall or the Vietnam War, the then Harvard professor was a prominent, often controversial voice in the seminal debates over the strategies for the nuclear age.

So long a public life lived on a global stage inevitably accumulates as many critics as admirers, let alone some infamous setbacks as well as the acclaimed successes.

And yet, having just turned 90, his views are still eagerly sought by diplomats, financiers and media pundits over almost every emerging flashpoint, from Syria and Iran to North Korea.

What accounts for such durability? In part you'd have to say the message; in part, the man.

Today, the Kissinger worldview is often used synonymously with realpolitik, that cold-eyed, unemotional view of statecraft famously practised by the likes of such 19th-century European diplomats as Metternich and Bismarck, and meaning the careful balancing of nations to avoid the domination of the continent by any one power.

In fact, Kissinger's strategies are an expanded version of that notion, which, it should be noted, did not prevent Europe's nations from competing to the point they toppled into the First World War.

Balancing power

Kissinger holds that the world not only needs to be in a power balance that prevents the hegemony of one superpower (even if that is the U.S., he would note).

But that competing nations must also share a mutual set of "rules and limits" that inhibits the kind of fight for dominance that might overturn international order.

A word in a president's ear. Kissinger chats with George W. Bush at a U.S.-China basketball game during the Beijing Olympics in 2008. (Danny Moloshok / Reuters)

In relations between the U.S. and China, for example, both sides will naturally compete economically. But Kissinger would warn against either one seeking an absolute advantage that might so unnerve the other that a serious conflict would emerge.

Kissinger, whose childhood was marred by his family's need to flee the rise of Nazi Germany in 1930s, believes that cautious diplomacy, even with notoriously anti-democratic regimes, can be the more virtuous route if it preserves international order. Statesmen must work with the imperfect foreign regimes they face, not with what they might ideally wish for.

In that admittedly pessimistic view, no one has a monopoly on virtue, there will be no all-out victors, and the historic national interests of all powers must be clearly recognized.

The result, however, can make for often unpalatable diplomacy between the good, the bad and the ugly.

For example, these days he strongly urges negotiations with Iran over its nuclear and Middle East ambitions, a view most U.S. conservatives abhor.

Passion for secrecy

To prevent public sentiment from getting in the way of diplomacy, Kissinger's approach was for highly secretive dealings, a source of much suspicion when he played the dominant role in U.S. foreign policy during the Richard Nixon and Gerald Ford years.

First as national security adviser, and later as secretary of state in the early and mid-1970s Kissinger choreographed an unprecedented drumbeat of coldly calculated — and convoluted — diplomatic offensives around the globe.

He boosted the policy of détente with the former Soviet Union through a series of confidence-building overtures that greatly eased Cold War tensions and permitted some of the early moves towards an arms control treaty.

Most famously, his secret 1972 negotiations with Beijing led to the almost unimagined visit of Richard Nixon, the arch cold warrior, to China, and the first beginnings of a historic Pacific rapprochement that reshaped the world.

To admirers, this was Kissinger's subtle masterpiece, one of those hinge-of-fate moves that terminated one historical era and gave birth to another.

Though weakened at home by anti-Vietnam War protests and Watergate, the U.S., over the course of a remarkable few years, was able to leverage this new relationship with China into a new balance of global giants.

This then permitted Kissinger to first negotiate the end to the long war in Vietnam (in 1975), and then to further draw a worried Soviet Union into deeper détente (the Helsinki accords of 1977).

Analytically timeless

Kissinger found himself under fire in these years from both the left, which viewed him as a warmongering defender of dictatorships, and from right-wing Republicans, especially Ronald Reagan-style cold warriors, who loathed him as a weak-kneed appeaser of world Communism.

Fundamentally, Kissinger offended the two traditional forces in U.S. diplomacy, both of which flow from the country's firm belief in its "exceptional" national virtue.

The isolationist wing distrusted such grubby moves to cement foreign alliances; while the idealistic crusaders for U.S. interventionism, to export American democracy and commercial values, were horrified by his dealings with Beijing and Moscow.

These days, many of Kissinger's admirers believe that one of his biggest contributions over the years was to force Americans to grow up and take a more sophisticated view of world — essentially to see it as a world that America can neither dominate as it once believed, nor withdraw from as it once hoped.

It is a legacy that can be seen today in President Barack Obama and a good part of the U.S. foreign policy establishment who seem to share Kissinger's general view of statecraft, particularly as regards China and Russia.

Even so, it has not stilled the harsh criticism that has continued to dog Kissinger's reputation for duplicity and amoral callousness in the way he conducted foreign affairs.

The late British-American writer Christopher Hitchens wrote a fiercely accusatory book demanding Kissinger be tried for "crimes against humanity" for his role in helping overthrow the elected socialist government of Chile in 1973 and for the years of oppression under General Augusto Pinochet that followed.

Both the destabilization of Chile (allegedly an effort to save it from Communism), and the secret bombing of Vietcong supply depots in Cambodia during the Vietnam War remain deep stains on the Kissinger record.

"The setbacks (he) encountered as a statesmen and the antagonism he engendered as a person, stemmed from the perceived amorality of his geopolitical calculations," wrote historian Walter Isaacson in his very balanced biography Kissinger.

And yet people still seek his advice because his worldview offers such a thoughtful framework to test certain strategies against.

There is also the man's intellectual brilliance, which even strong critics concede, and his remarkable ability to vividly conceptualize patterns and relationships among different events, and to make the hard choices abundantly clear.

As foreign affairs analyst Robert Kaplan wrote recently in a glowing review of Kissinger's legacy for The Atlantic magazine, Kissinger's classic realism is "emotionally unsatisfying but analytically timeless."

I suppose that's why in any world crisis there always does come that point when, like it or not, I still feel the old need to at least check in with what Henry Kissinger has to say.