As Israeli ground forces cross over into Lebanon in an effort to destroy Hizbullah's capacity to fire rockets and missiles at Israeli civilians, the president of Lebanon Emile Lahoud is taking sides and flexing his nation's muscle for the first time. He told a German magazine that he "respects Hizbullah" and its terrorist leader Hassan Nasrallah, and he has threatened to turn the Lebanese army against Israel. This would cause a self-inflicted, and perhaps lethal, wound on Lebanon. Militias associated with several Lebanese political parties -including the party of the speaker of the parliament and of the Lebanese Communist Party - have already joined Hizbullah in attacking Israel. Up until now, the Lebanese government has claimed that it has no real army, not even one capable of controlling the Hizbullah terrorists who have "occupied" southern Lebanon. It has claimed an inability to enforce UN resolution 1559 which, adopted by the Security Council in 2004, "calls for the disbanding and disarmament of all Lebanese and non-Lebanese militias," meaning Hizbullah. And it has disclaimed any responsibility for Hizbullah atrocities against Israeli civilians and kidnapped soldiers. NOW, ALL of a sudden, its tune has changed. Suddenly its army is strong enough to take on Israel, and it is willing to collaborate with the terrorist organization from which it previously sought to disassociate itself. Nor can the Lebanese government plausibly argue that this about-face has been caused by the Israeli decision to have its ground troops cross over into Lebanon. Intelligence sources have confirmed that elements of the Lebanese army have been working with Hizbullah terrorists from the beginning of this conflict, providing the terrorists with electronic, radar and other logistical supports. The Lebanese government is not as innocent - not as much of a victim - as it has gotten the international community and the media to believe. It is playing a dangerous and two-faced game of both sides against the middle. Large segments of its Islamic population in Lebanon support Hizbullah, while much of its Christian population is terrified of the radical Islamic goals of Hizbullah but too intimidated to speak out. Israel and the United States do not want to turn Lebanese moderates into anti-Israel and anti-American zealots, but the tragic reality is that moderates have little influence in Lebanon these days. Israel made a serious blunder by worrying too much about the impact its military actions might have on Lebanese public opinion, and was consequently quite tentative in its initial response to Hizbullah rockets. If this decision was made for military reasons, I would have insufficient expertise to question it. But it appears that military considerations may have been compromised in order to influence Lebanese public opinion. If that was the reason, it was self-defeating. ISRAEL WILL be criticized regardless of what it does or refrains from doing. If it succeeds in destroying Hizbullah, it will be criticized for the civilian deaths it caused, even though the civilian deaths are the fault of Hizbullah for operating from civilian population centers. If Israel fails, it will be criticized for leaving the poor Lebanese in the grip of an emboldened Hizbullah. Israel was attacked from areas that it does not occupy. It gave up this land as part of what peace-lovers in Israel believed was a swap of "land for peace." But Hizbullah and Hamas turned it into "land for rocket launching and kidnapping." Israel is entitled to fight back and defend its citizens from aggression, as every other democracy would do. The media and international organizations are focused on the plight of the Lebanese who have had to leave their homes, paying little attention to the thousands of Israelis who have had to leave their homes in Haifa, Safed, Nazareth and other cities in range of Hizbullah rockets. They are obsessed with comparative body counts, as Hizbullah complains that more Lebanese than Israelis have been killed. Would they be satisfied if more Israelis had been killed? Would that render Israel's actions more proportional? Should Israel be blamed for having built bomb shelters for its civilians, while Hizbullah has built bunkers only for its leaders, leaving civilians exposed? Should Hizbullah be given credit for deliberately increasing the number of civilian deaths on both sides by firing its anti-personnel rockets into densely populated areas and from densely populated areas? Should Israel be blamed because many Israelis left their homes in dangerous areas while many Lebanese decided to remain in war zones in order to serve as human shields for the terrorists? HIZBULLAH IS on the wrong side of morality, international law and human rights. If the Lebanese army now joins it in fighting against Israel's efforts to stop terrorism, it will become complicit with terrorism and a legitimate military target. It will share responsibility for the deaths of its own civilians. It would be wise and moral for it to stay out of the fight and preserve its firepower to use against the real enemy of Lebanese democracy - Hizbullah. The writer is a professor of law at Harvard and the author of Preemption: A Knife that Cuts Both Ways.