Israel is the strongest military power in the Middle East. The gap between its intelligence and operational capabilities and the way it could have employed these capabilities to help the Syrians was starkly illustrated this week: On Sunday, Mr. Assad appears to have once again used poison gas on his own citizens. He knew that the international community would discover this, but it was more urgent for him to defeat the rebels in the Damascus suburb of Douma before a cease-fire agreement. He also knew very well, based on his experience in recent years, that his war crimes would be likely to go without serious punishment.

One day after that gas attack, Israel did carry out a strike in Syria. But the operation had nothing to do with the atrocity in Douma; its timing was coincidental. Israel struck a Syrian air base, T4, at which various aircraft were reportedly operated by Iran’s Islamic Revolutionary Guard Corps. According to reports, seven Iranians were killed.

“Israel has several red lines, which it is not prepared to allow to be crossed, including the transfer of sophisticated weapons or chemical weapons to Hezbollah,” Avigdor Lieberman, the minister of defense, told a session of the Knesset’s Foreign Affairs and Defense Committee in December 2016. Then he added: “We have no interest in intervening in Syria’s civil war.”

Israel is not alone. Much of the responsibility for what is happening in Syria can be laid on the doorstep of the United States, which has largely stood by as hundreds of thousands of Syrians have been massacred. President Barack Obama decided not to act against Mr. Assad, even when the Syrian president crossed the red lines that Mr. Obama himself had drawn. President Trump has continued down this path, apart from a token salvo of Tomahawk missiles a year ago, which certainly did not deter Mr. Assad. The United States now seems poised to strike once more, but will it do enough to change the Syrian president’s calculus?

Years ago, Israel could easily have forced Mr. Assad’s military, stretched to its utmost trying to suppress the rebellion, to halt at least some of its actions against civilians. The Israeli armed forces could have stopped the Syrian Army or at least coerced it into declaring a “safe zone” for refugees with a no-fly zone over it. The claims that this would have entailed a complex or even impossible military operation are rebuffed by the facts: Israel has carried out hundreds of attacks in Syria during this same period.

Syria, in most cases, did not even attempt to defend itself from these attacks. Mr. Assad’s generals seem fully aware of their military’s inferiority. They also knew that by responding with force, they would only bring on themselves more devastation. On Wednesday, a high-ranking Israeli source told me that if Iran were to attempt to retaliate for the attack on its base in Syria, “both the Assad regime and Assad himself would vanish from the face of the earth.” Imagine if a threat like this were uttered about saving innocent lives.

Israel has had excellent reasons for not intervening: Operating overtly in a neighboring country would appear as an intolerable interference in Arab affairs to the Arab world, which already hates Israel. And of course, Israel has numerous other security challenges. Moreover, since the West neglected Syria for so long, the situation has grown far more complex: Iran, Hezbollah and Russia are in control. Any Israeli operation now would be far more complicated and would have to be carried out in cooperation with the United States.

The arguments against an Israeli action are based on weighty considerations. But were there not weighty considerations in the 1940s that stopped the Allies from coming to the aid of Europe’s Jews? And if these explanations were not legitimate then — and we know now that they were not — what about today? What is happening in Syria is not the same as the gas chambers of Auschwitz. But does Israel not have an added obligation, over and above that of the other countries, to do something for nations facing genocide and war crimes, especially when they are right on its northern border, and it has proved several times that it is able to do so?