Two weeks ago, James Mattis, the U.S. secretary of defense, attempted to justify the provision of U.S. arms to Ukraine. “Defensive arms,” he said, “are not provocative unless you are the aggressor.” The claim was as banal as it was wrong.

Secretary Mattis’s statement made for good politics, and it also makes a degree of intuitive sense. But three generations of students of conflict who have studied “the security dilemma” know it is not, in fact, the case. All too often, nations act in such a way—building up big armies or navies—that they assume will better protect them from their adversaries. What they fail to realize is that sometimes their adversaries will view these “preventive” or “defensive” actions as quite aggressive, in fact, and will make conflict more, not less, likely.

This matters because the next big war in the Middle East, between Hezbollah and Israel, will begin because Hezbollah has vastly increased the size and sophistication of its arms in Lebanon despite clear and consistent warnings from Israel and the international community not to do so.

We Americans are forever viewing the strategies of our adversaries through rose-colored lenses. But just like us, our adversaries sometimes do really stupid things.

Vladimir Putin, for example, has developed a reputation as a master strategist by successfully meddling in the U.S. elections. But it’s also quite reasonable to ask whether irrevocably poisoning relations with two generations of Americans in the process was actually quite dumb, strategically speaking.

Washington and its allies often view Iran, Hezbollah’s sponsors, in a similar way—which is to say we often give them more credit for being master strategists than they deserve. Tehran is so patient, America’s Gulf Arab partners lament. Well, yes, but its slow, steady strategy to arm, train, and equip sectarian Shia militias across the Middle East is a recipe for sowing instability in its own neighborhood. The Iranian regime, which can be tactically very clever, is creating states in which it will always have powerful proxies and partners, but will never have the kind of peace in which the vast (and growing) arsenals of the region are not pointed at Tehran.

So, despite the mystique that surrounds Hezbollah—especially since the 34-day war in the summer of 2006 in which it defeated Israel—don’t underestimate its capacity to do something equally dumb.

Hezbollah was the only major militia allowed to retain its weapons upon the conclusion of the Lebanese Civil War, so it should be no surprise that it has grown to dominate Lebanese politics in the years since. Repeatedly, Hezbollah has claimed it is importing advanced rockets and missiles into Lebanon—and is reportedly now constructing such weapons in Lebanon itself—to defend Lebanon from Israel.

Israel, after all, has carried out offensive actions in Lebanese territory since the late 1960s and launched full-fledged invasions in both 1978 and 1982. Following the 1982 invasion, Israeli soldiers did not fully withdraw until 2000. But withdraw they did, and Hezbollah’s genius has been to convince the Lebanese people of the necessity of its arms despite that withdrawal.

After its humiliating defeat of Israel in 2006, and despite the steadily improving capabilities of the Lebanese military, Hezbollah convinced many Lebanese that only it could truly defend Lebanon from Israel. Never mind the fact that it was a wrong-headed Hezbollah raid into Israeli territory that kicked off the fighting. Similarly, a broad cross-section of Lebanon’s population—including much of its Christian community—buys into Hezbollah’s argument that Hezbollah and its arms also protect Lebanon from the crazed Sunni extremists it has been fighting in Syria and who threaten Lebanon. And there’s some truth to this! Hezbollah is indeed fighting crazed Sunni extremists in Syria.

But Hezbollah has hardly been a dispassionate referee amid the sectarian civil strife that has infected the Middle East since the U.S. invasion of Iraq in 2003. In 2008, Hezbollah, sensing its domestic power in Lebanon waning, turned its arms against other Lebanese in a spasm of street fighting that further inflamed the country’s frustrated Sunni communities. And in Syria’s civil war, Hezbollah has been a key actor in a regime-led strategy that first systematically eliminated any non-extremist elements in the Syrian opposition, leaving the people of Syria and the international community with an ugly binary choice between the regime on the one hand and groups like al-Qaeda and the Islamic State on the other.

Tactically, that was clever. Strategically, that was a recipe for a radicalized and perpetually aggressive Sunni majority that will terrorize Syria—and, perhaps, Lebanon—for decades.

Furthermore, Hezbollah has also backed Iran’s efforts to arm, train, and equip sectarian Shia militias in not only Syria but also Iraq and Yemen—a development that has contributed to the sectarian backlash that helped produce not only ISIS but also the disastrous conflict in Yemen.

Blowback was inevitable. For the past several years, Israel has been warning Hezbollah, first indirectly and then publicly, about its “red lines” in Syria, which included the transfer of sophisticated weapons from Syria into Lebanon. Outgoing Israeli air force chief Amir Eshel later revealed Israel had struck Hezbollah arms caches and convoys in Syria over 100 times.

But Israel knows it cannot intercept each and every shipment of arms. Sometimes the weather is too poor, or the intelligence too fuzzy, to act. And now Hezbollah is reportedly developing indigenous arms-making capabilities that will render cross-border shipments of advanced weaponry less necessary.

So, for nearly two years now, Israeli military and intelligence officials have been warning every American official who comes through Tel Aviv and Jerusalem that the next war is coming. Israel has methodically prepared its allies—and most especially the Americans— for a very, very ugly war on the horizon. I don’t claim to know the precise spark that will ignite the next conflict, but Israel has been clear about the things it considers to be casus belli, and Hezbollah has consistently ignored them.

I shudder to think what the next conflict will look like. Since 1993, each clash between Israel and Hezbollah in Lebanon has been successively more violent. 1993’s Operation “Accountability” in southern Lebanon involved mostly air and artillery. 1996 brought Operation “Grapes of Wrath,” and the destruction of Lebanese civilian infrastructure beyond southern Lebanon. The war in 2006 leveled entire neighborhoods in Beirut, and led to the evacuation of hundreds of thousands of civilians in both Lebanon and Israel.

Now, especially since Hezbollah has dispersed its arsenal across Lebanon, the entire country will burn, and Israel will suffer mightily. Not one but two U.S. allies—for the United States has invested heavily in the Lebanese army, which Israel will almost certainly treat as hostile, and which will almost certainly attempt to defend Lebanese territory—will suffer.

And for what? What will Hezbollah have accomplished, other than the destruction of its villages and cities? It has duped its young fighters, who have been fighting the Israeli project of late in such hotbeds of Zionism as Aleppo and Deir az-Zour, into thinking Israel is weak.

Hezbollah has grossly underestimated Israel, a mistake that will prove costly. Hundreds and perhaps thousands of Israelis could die in another conflict, but Israel isn’t going anywhere. This is existential for them. It will be the Lebanese who suffer immeasurably more.

If only Hezbollah could realize that, rather than pursue its present course, so much pain could be avoided.