Sometimes long-festering problems collide — and explode — in a single memorable year. We can go as far back as the fifth century B.C. to see this phenomenon — and we may see it again in 2010.

In 480 B.C., a decade of Aegean tension culminated in the Persian invasion of Greece. Nothing seemed able to stop the onslaught of King Xerxes as he broke through the pass of Thermopylae — until the Greeks under Themistocles rallied at the sea battle of Salamis and saved the West.

In A.D. 69, the Roman Empire was tottering on its very foundations. Rome had been rocked by decades of corruption, assassinations, coups and military revolts. By the end of 69, Vespasian — the fourth emperor that year! — brought sanity back to Roman government.

Fast-forward to the modern era. The rise of fascism erupted into war and conquest in 1939. That year, Franco's Nationalists won the civil war in Spain. The Soviet Union fought Japan in a border war — during which it signed the Molotov-Ribbentrop non-aggression pact with Hitler's Germany. Weeks later, the Nazi invasion of Poland marked the start of the Second World War.

Events in 1939 alone did not cause the outbreak of the global conflict. Rather, it followed from years of bad ideas like serial appeasement of Hitler, the near-disarmament of Western democracies and flirtation with pacifism. This behavior had inadvertently sent a global message: Britain, France and the United States were unwilling and unable to meet the challenge of totalitarianism. And so dictators called their bluff in 1939 and began to move.

Closer to the present, 1979 was another climactic year. Jimmy Carter's prior years of sermonizing about American bad habits had convinced many of the world's bad actors that it was time to press forward their regional agendas without fear of American reaction.

Daniel Ortega's Sandinistas took control of Nicaragua. The Iranian Revolution triggered an oil panic. A global energy crisis followed. Islamic terrorists took American hostages at the U.S. embassy in Tehran. The Soviet Union's Red Army entered Afghanistan.

2010 may turn out to be a similar year of destiny. In 2009, the United States gave Iran at least four deadlines to stop its nuclear program. All were ignored. Does an emboldened theocracy believe this now is the year to become nuclear and change the entire strategic makeup of the Middle East?

For much of 2009, the Obama administration boasted that it would shut down the Guantanamo Bay terrorist detention facility, despite having no final idea of where or what to do with all the detainees — many from terror-infested Yemen.

We renounced prior notions of a "war on terror." We reiterated that the now-quiet Iraq war had been a mistake. We apologized to the Islamic world for purported past sins, while inflating Muslim achievements.

After months of hesitation, in Janus-fashion we both announced we were sending more troops to Afghanistan and promised to start soon bringing them home.

All of this has not been lost on Islamists. In general, al-Qaida interprets our outreach as a sign of moral weakness. Since 9/11, more than one-third of all terrorism-related incidents in the United States occurred in 2009 alone.

In 2010, our year of decision, events may come to a head and overwhelm the existing American-led global order unless President Obama can galvanize Western allies to meet the mounting danger.