So much has happened since the terrorist attacks on New York and Washington on September 11, 2001, that the events now seem both very distant and perhaps less consequential than they appeared at the time. However, both impressions are misleading.

Twenty-five years is a considerable period in a human life, but little more than a moment in history, and however turbulent the past quarter-century has been, the significance of September 11 is only now becoming fully apparent.

Looking back, and setting aside the emotions of the time, what did the attacks actually mean for the international system?

Above all, they brought an end to a decade of virtually unchallenged US hegemony in world affairs.

The significance was partly symbolic in that the attackers struck at the two most obvious centers of American power, the World Trade Center represented its financial might, while the Pentagon represented its military strength. The post-Cold War period, during which the United States had come to believe that no fundamental threat remained to its security or interests, was over.

From Washington’s perspective, the decade had begun with the almost miraculous disappearance of its principal geopolitical adversary, the Soviet Union, yet it ended with the realization that danger could emerge suddenly from almost anywhere.

The fact that most Americans had previously known little about those who attacked them only deepened the shock and contributed to the impulsiveness of the response.

The End of History, proclaimed by Francis Fukuyama as the Cold War was drawing to a close, was followed barely a decade later by history’s turbulent and violent return

Washington’s reaction set in motion a global realignment whose consequences are now much easier to see. Whether Osama bin Laden possessed some ingenious long-term strategy for undermining American power is impossible to know, but the result was obvious as the United States embarked upon a course that gradually weakened its ability to manage international affairs.

George W. Bush’s administration, strongly influenced by neoconservative thinking, was caught off guard by September 11, so its response rested on two broad assumptions.

The first was that threats to America could originate anywhere, not merely from hostile states but from non-state groups newly capable of inflicting enormous damage, therefore such threats had to be confronted wherever they emerged, primarily through the overwhelming military power available to the United States.

The second assumption was ideological, given liberal theory held that democracies didn’t wage war against one another, meaning if that were true, then the more democracies there were, the safer the United States would become.

From this emerged the increasingly assertive promotion of democracy abroad, which at times it became openly aggressive.

Its methods ranged from military interventions in Afghanistan and Iraq to political and financial support for democratic movements in the former Soviet Union and pressure for liberal political reforms across the Middle East and Libya later became another example of Western military intervention producing consequences far beyond those originally anticipated.

Some episodes from that period are now half-forgotten, such as when Washington strongly encouraged elections in the Palestinian territories despite warnings from regional actors about the likely consequences. Naturally, Hamas subsequently won the 2006 Palestinian legislative election, dramatically altering Palestinian politics.

The same confidence in political transformation characterized the American response to the Arab Spring as existing political balances across the Middle East were overturned, often without anything stable emerging to replace them.

The consequences of the 2003 invasion of Iraq were greater still. The overthrow of Saddam Hussein destroyed a major component of the regional balance that had developed during the 20th century and unleashed forces that proved extremely difficult to contain.

America became trapped in what its own politicians would eventually call endless wars and these conflicts contributed to polarization inside the United States while attempts to export democracy damaged the reputation of democracy itself.

In Iraq, this was compounded by the widespread belief that American strategic and economic interests, including oil, were inseparable from the intervention.

The broader results were remarkable with prolonged wars, a destabilized Middle East and the expansion of radical Islamist movements. The architects of September 11 could scarcely have hoped for more when they planned their attack given relations between the West and much of the Muslim world also deteriorated.

President Bush’s early reference to a crusade against terrorism was probably an unfortunate choice of words rather than a deliberate provocation, but Islamist movements eagerly seized upon it as confirmation of their argument that Western hostility towards the Muslim world had changed little since the Crusades.

Washington subsequently tried to repair those relations, but the direction had already been set.

The destruction of Iraq’s complicated internal and regional balance created opportunities for radical movements while contributing to an image of the United States as an enormously powerful but unpredictable force in Middle Eastern affairs. As a result, for governments in the region, maintaining some distance from Washington increasingly appeared prudent.

September 11 also transformed the United States itself.

The War on Terror was conceived as a global campaign to protect American national security, but its domestic consequences were profound as surveillance and state control expanded dramatically, whether in personal communications or financial transactions.

Although the original justification was terrorism, over the following quarter-century, new reasons for expanding these mechanisms appeared while technologies changed and political priorities shifted, but the general direction towards greater monitoring and control remained.

Nor did the old problem of double standards disappear because, despite declarations of zero tolerance for terrorism, the familiar distinction between one side’s terrorist and another’s freedom fighter survived. This ambiguity becomes particularly apparent when the interests of major powers collide, whether those powers are the United States, Russia or China.

A quarter-century after September 11, the world bears remarkably little resemblance to the one that existed on its eve, but, of course, much of what followed would probably have happened anyway. International systems change because of forces far larger than any single event and China’s rise, Russia’s reassertion of its interests, technological transformation and the relative decline of Western economic predominance cannot simply be attributed to the attacks of 2001.

But September 11 was nevertheless a turning point as the attacks ended the extraordinary post-Cold War interlude in which American predominance appeared almost uncontested, and, more importantly, Washington’s response accelerated the processes that brought that period to a close.

The movement away from the unipolar order didn’t happen overnight, but began gradually, gathered momentum through wars, crises and the emergence of alternative centers of power, and continues today.

What will replace it remains unclear, given the formation of another stable international order may take many years and whether that system proves preferable to the bipolar confrontation of the second half of the 20th century, or to the brief period of American hegemony that followed it, is impossible to know.

But one thing now seems clearer than it did on September 11, 2001 in that the attacks didn’t merely interrupt the post-Cold War order, but marked the beginning of its end.