Historians like to joke that we know a lot about big wars. For example, odd-numbered wars, like World War I, always start in the Balkans, whereas even-numbered ones, like World War II, begin in Poland.

The point of the joke is that every war is unique, and so sheds only hazy light on today. The impulse to learn from the history of the First World War, which began 100 years ago, is difficult to resist, but history cannot tell us with certainty what we want to know.

In his history of the Great War, Winston Churchill was asked whether Britain could, by a dedicated act of friendship, have reconciled France and Germany and averted the war. He replied that he could not tell. All he knew was that he had done his best, and that his nation had survived. That is the wisdom of Olympus.

But those of us less wise than Churchill will be quicker to compare 1914 and 2014. And the two eras do have disturbing features in common. There was no one single factor that made 1914 particularly dangerous: It was a mix of elements, combined with bad luck, that made the war.

Without the great power rivalries that defined the age, the war could not have happened: A big war needs big powers. In the 1990s, many believed the era of such rivalries ended with the Cold War. Today, facing a revanchist Russia and a surging China, we have less cause for such naivete.

But not all the great powers in 1914 were genuinely great: the Austro-Hungarian Empire feared it was falling apart. Declining autocracies can be more dangerous in the short run than rising ones, because they are more inclined to take risks. And Russia has been taking a lot of risks recently.

advertisement | advertise on newsday

The reason Austria-Hungary feared for its future was the rise of ethnic politics in the Balkans. Today, we see religious as much as ethnic politics, but the effect is the same: Many nations around the world today, from Nigeria to Ukraine, look more like falling apart than pulling it together.

And finally, there is terrorism, which sparked the Great War with an assassination in Sarajevo. What is dangerous about terrorism is not simply that it kills the innocent. It is that, today as in 1914, terrorism is often a tool of nations: Those rockets striking Israel come from Iran. And that raises the stakes for everyone.

But the problem with comparisons to 1914 is that, as Churchill once quipped, things never happen the same way twice, because otherwise life would be too easy. The best you can do is to use history to improve the odds that things will go your way. That's also what deterring war is about: It gives peace a fighting chance.

And tragically, that was what Britain failed to do. The best chance to avert the First World War passed well before 1914. If Britain had been willing to build a large land army, and to pledge itself firmly to the defense of France, Germany might have been deterred. It was no more than a chance -- but it was a chance.

But Britain wasn't willing to do this. It intervened in force only after the war started. In 1939, it made the opposite mistake: It barely got involved, France fell, and Britain had to face Germany alone. The third time, Britain, working with the United States, got it right, by deterring the USSR before the Cold War turned hot.

Today, President Barack Obama lashes out at critics of his vacillating foreign policy as adventurers. But in 1914, liberal Britain could not deter because it would not firmly commit to defending its ally. In a crisis, strength offers no certainties, but weakness will never deter. And in 1914, a failure of deterrence may very well have cost the peace of the world.

Ted R. Bromund is a senior research fellow in the Heritage Foundation's Thatcher Center for Freedom.