In the past 40 years, the United States has intervened to go after autocrats in Afghanistan, Grenada, Haiti, Iraq, Libya, Panama, Somalia, and Serbia. We have attacked by air, by land, and by a combination of both. In the post-Vietnam, post–Cold War era, are there any rules to guide us about any action envisioned against Syria or Iran — patterns known equally to our enemies?

1. The target cannot have nuclear weapons. Strongmen in Pakistan and North Korea by virtue of their nukes are exempt from American reaction (unlike Syria or, at present, Iran) — unless they directly threaten our existence or that of our allies. With the end of the Cold War, many rogue states lost the Soviet nuclear umbrella and are still scrambling to acquire their own nuclear weapons to ensure them deterrence, especially against the United States, which has not yet invaded a nuclear nation.

2. We do not attack large countries. About 30 million or so — roughly the population of Iraq or Afghanistan — is the upper limit. That criterion suggests that we will not ourselves seek regime change in Iran (population: 65 million) through force — a different case from punitive bombing or preemptive air attacks on its nuclear facilities.

3. The target should not directly border either Russia or China. We violated this commandment in Afghanistan, apparently encouraged by the global climate of goodwill toward America after 9/11, the short and mountainous Chinese border, and the fact that China shares our fear of radical Islam. But otherwise, after Vietnam and the Cold War, the former Soviet republics, North Korea, Tibet, and the countries of Southeast Asia will always be off-limits to U.S. intervention.

4. U.N. sanction and U.S. congressional approval, however praised and sometimes sought, seem irrelevant. We obtained neither before bombing Serbia, the former but not the latter in Libya, and the latter but not the former in Iraq. We obtained both for Gulf War I, but neither for Panama or for Grenada.

5. Africa seems exempt. Tens of thousands perished in Congo, Darfur, and Rwanda. Africa has oil. No matter. Somalia is as much Middle Eastern as African, and our intervention there was a particularly half-hearted affair. In Africa, even genocide is not a reason for U.S. military intervention — quite in contrast to Serbia, where NATO finally intervened. Idealism is often as praised as it is subordinated to realist concerns.

6. We often intervene in Central America and the Caribbean — the Dominican Republic, Grenada, Haiti, Panama — but are less likely to do so in South America, where the politics are riskier, the distances greater, and the nations larger and stronger.

7. Intervention is mostly a bipartisan affair. Democrats went into Haiti, Libya, Serbia, and Somalia, Republicans into Afghanistan, Grenada, Panama, Iraq, and Libya. Republicans may have intervened a little more since Vietnam, but then there have been more years of Republican administrations. Anti-war protests are usually aimed at Republicans, rarely at Democrats, who enjoy far more latitude in the use of force.

8. There is no consistent or predictable rationale for invading a country; it can be supposed national interest and/or oil (Iraq, Libya), “humanitarian” considerations (Haiti, Serbia, Somalia), spheres of interest (Grenada, Panama), or simple retaliation (Afghanistan).

9. The insertion of ground troops is necessary to create postwar governments (Afghanistan, Iraq, Serbia, etc.); without them we have little influence (Libya).

10. The target is usually a government rather than gangs, tribes, or terrorists; if it is one of the latter, either we do not go in to remove those in control, whatever the provocation (Lebanon), or we fail when we do (Haiti, Somalia). The verdict on Afghanistan is still out.

11. We are adept at removing dictators (Afghanistan, Grenada, Iraq, Libya, Panama, Serbia), but less so at fostering calm in their wake (Afghanistan, Iraq, Libya).

12. The American people usually favor intervention at the outset, but regret it when hundreds of Americans are killed, or violence continues. Those who most assiduously demanded action are most likely to blame the leaders who followed their advice, apparently embarrassed when violence continues and our losses mount.

13. Russia and China almost always oppose our intervention. nations that support our intervention usually do so privately — and publicly only to the degree post facto that it is clear that we succeeded quickly and without much turmoil.

14. The U.N. has far more problems with removing genocidal dictators than with allowing them to perpetuate genocide.

15. No intervention provides much of a model for any other.

Based on these rules, we can make two general observations about Syria and Iran. In Syria, the U.S., on proper humanitarian grounds, could easily intervene through air power alone — without either congressional or U.N. sanction — to so weaken the non-nuclear Assad regime that, as happened in Serbia and Libya, it would surely and quickly implode. That said, we probably will not, given that such action would offend China and Russia, would not ensure quiet or stability in the aftermath, be soon criticized by those pundits who originally urged us to go in, and in six months be either unappreciated or overtly criticized by nations that had initially demanded that we do something to stop the slaughter.

As far as Iran goes, based on past precedents, there is zero chance that the United States would ever intervene to change the government, either on the ground or by an extended bombing campaign — and only a slight chance we will preempt by bombing suspected Iranian nuclear facilities.

— NRO contributor Victor Davis Hanson is a senior fellow at the Hoover Institution and the author most recently of the just-released The End of Sparta, a novel about ancient freedom.