The prospect of Congressional approval for a U.S. attack on Syria is probably good news for the world economy and financial markets, since the impact on the oil price of an intense but strictly time-limited military action is likely to be a classic case of “buy on the rumor and sell on the news.” History suggests that the moment U.S. bombs start raining down on Syria, oil prices will pull back and stock markets around the world will rise. But what about the bigger picture? How will a U.S. bombing campaign affect the stability of the Middle East and global geopolitics?

To consider these questions it helps to recall that the main principle underlying the United Nations Charter is non-intervention by foreign governments in the affairs of sovereign states. Morally, this principle is hard to justify. It conflicts with the “duty to protect” civilians from barbarous treatment by their own rulers, which Western governments have invoked when crossing international borders in response to massacres in former Yugoslavia, Sudan and Sierra Leone — and should have invoked, with hindsight, to stop Hitler in the 1930s and prevent the Rwandan genocide in 1994.

Why, then, is non-intervention still recognized as the bedrock of modern international law? A standard answer is the Peace of Westphalia — a series of treaties in 17th century Europe that legally enshrined national self-determination and inviolability of borders for the first time. But why should the world today still be bound by these 400-year-old ideas?

Maybe because the Peace of Westphalia emerged from what was perhaps the bloodiest war in the history of Europe, a war which unfortunately has terrifying similarities to events now engulfing the Middle East. The Thirty Years’ War of 1618-48 was a series of religious and sectarian struggles, mainly between Protestant and Catholic rulers in small German principalities, that sucked in all of the great Continental powers and probably killed more people, proportionately to population, than any European conflict up to World War Two. This war was marked by horrific massacres of civilian populations and by looting, rape, torture and genocide justified by religious doctrine. It wiped out an estimated 25 to 40 percent of the people in what is now Germany, with some German states such as Brandenburg and Wuerttemberg allegedly losing as much as two-thirds of their populations.

Could there be some instructive parallels between the Thirty Years’ War and the religious killing today in Syria, Iraq, Lebanon, Libya, Afghanistan and Pakistan, with proxy backing from Saudi Arabia and Iran and less directly from the U.S., Egypt and Israel? The obvious parallel is that Sunnis and Shi’ites have already been fighting for 30 years, starting with the Iran-Iraq war of the 1980s — and broader religious warfare has gone on much longer if we include the Israeli conflict. So U.S. bombing Syria is unlikely to make the Middle East either more or less stable; it will merely continue the status quo. But even if the warfare continues, might it become less barbaric?

This seems unlikely. Punishing Bashar al-Assad for using chemical weapons and at the same time disavowing any attempt at “regime change” might even signal that massacring civilians is acceptable, provided the killing is done with bullets, bombs or beheadings, and not poison gas. In fact, gaining an implicit sanction for “conventional” killing might even have been Assad’s secret objective in provocatively crossing President Obama’s “red line” on chemical weapons.

But even if limited bombing fails to prevent future massacres, might it shorten the conflict by drawing the U.S. deeper into Syria, leading to Assad’s overthrow? This, too, is unlikely. A U.S. ground invasion in the Middle East is politically unthinkable after the Iraq experience. But without a U.S. military presence, regime change in Syria would probably bring in a new government as hostile to Western interests and perhaps as murderous as Assad’s. Even a miraculous flowering of democracy might not produce better results. Judging by events in Iraq and Egypt, the Sunni majority in Syria would probably elect a fundamentalist regime, backed by Saudi Arabia and al Qaeda, that would want to massacre Shi’ite and Alawite “apostates” in revenge for the Sunnis massacred by Assad’s Alawites, supported by Shi’ite Iran.

Which brings us back to possible parallels with Europe’s Thirty Years’ War. Why did a war apparently motivated by religious differences — not only between Catholics and Protestants but also between Protestant Lutherans and Calvinists — kill more people in Europe than previous conflicts caused by economic interests and territorial disputes?

Partly because religious fanaticism can inspire hatred, legitimize violence, turn cruelty into self-righteousness and devalue the lives of unbelievers. But probably more important was the way that religion could disguise the true motivations — economic, territorial or dynastic — of outside interests exploiting the anarchy in central Europe for their own gains. What prolonged the religious wars in Europe for so many decades was not just spiritual fanaticism. It was the persistent intervention of external powers — Austria, Spain, France, Sweden, the Papacy, Turkey and Denmark — that found irresistible opportunities to fight proxy wars on German territory, instead of their own land.

These external powers created an unstoppable war machine, by feeding in mercenaries, money and weapons into the collapsing German principalities long after their domestic human and economic resources were exhausted. Without external support, the feeble German princes might have fought themselves to a standstill in years or perhaps even months, rather than decades — and would have found it physically impossible to keep fighting after so many of their citizens had been killed. But as long as the money and mercenaries from Madrid, Paris, Vienna or Rome kept flowing, the killing just went on and on.

It was only after all the great powers of Europe had gone bankrupt, that the fighting in Germany gradually subsided and the Peace of Westphalia was agreed. Meanwhile, England, the one major nation that stayed out of the conflict, emerged as the world’s dominant economy and superpower.

It is better to learn from history than to repeat it.

PHOTO: Smoke rises as seen from the Aleppo district of Salaheddine September 4, 2013. REUTERS/Ammar Abdullah