The death of every child - Lebanese in Qana or Israeli in Haifa - is tragic. A just and lasting peace can only be achieved by addressing the underlying problems.

Israel has every right to defend itself against terrorism, and the Israelis can count on the United States. We must put Hamas, Hezbollah, Iran and Syria on notice that wiping Israel off the map and killing civilians are not the legitimate ends and means of foreign policy.

At the same time, the Lebanese must know that Americans also care about protecting civilians and preserving their fragile democracy. We must make clear that we remain committed to creating the base for lasting peace.

That's why we must work urgently, including through the United Nations, to achieve a sustainable cessation of hostilities that ensures Israel's security and Lebanon's territorial sovereignty, as well as an end to the threat posed by Hezbollah's state-within-a-state on Israel's northern border.

I once visited an Israeli town near the Lebanese border, Kiryat Shemona and went into shelters where terrified Israeli kids go when the rockets start falling. Now that Israel has withdrawn to an internationally-recognized border with Lebanon, why is Hezbollah still firing rockets?

This is not merely Israel's problem with its neighbors - this is a democracy defending itself from extremists determined to launch missiles against Israel's cities, kidnap its soldiers and cast a shadow of fear over the entire state.

This should be a wake-up call to refocus our energies on creating a zero-tolerance policy for terrorism and fostering the conditions necessary for lasting peace.

We can't rely only on crisis diplomacy. There's no better way to ensure another crisis another day. We need sustained, preventative diplomacy to address underlying problems before they explode.

That means putting an end to state sponsorship of terrorism by Iran and Syria. And that requires a renewed commitment to work tirelessly to achieve a lasting peace.

We have seen what happens when Israel is left to tackle these problems alone. In September 2004, the world community ordered Hezbollah to disarm. But for the last two years, Hezbollah has not been forced to comply. If we do not disarm these groups through diplomatic pressure, then they will be disarmed militarily - at great human cost.

We stood with Israel when it withdrew from Lebanon and Gaza. We shared Israel's hope that freedom might inspire its neighbors to become partners in peace. And we stand with Israel now. Hamas and Hezbollah have sent Israel a terrible message: When you withdraw from occupied land - when you do the right thing - terrorists will simply step up to the new border and start firing from that much closer.

We must address root causes and help Israel do the same. As long as Israel's border cities live under threat, any peace will be fragile and short-lived. And we can't let Iran and Syria stoke these simmering crises anytime they need a get-out-of-jail-free card.

This situation - and really the entire world - is crying out for U.S. leadership. We need to pursue smart, muscular diplomatic solutions to problems too complex to be solved through force alone.

After 9/11, the world condemned terror in one voice. We must reconsecrate that promise and make it a reality, in Israel and across the world.

John Kerry is a U.S. senator from Massachusetts.