As I write these words, several hours after its onset, the ceasefire in Lebanon is holding. Another hour or day from now, thus may or may not still be the case.

But let’s assume that the ceasefire does hold — or that if it doesn’t, it will eventually be restored.What happens now?

This depends of course on Israel.And on Hezbollah.And on the Lebanese government. And on Syria. And on Iran. And on Europe. And on America. In short, it’s going to be very complicated.

A best-case scenario would go as follows:

As the Israeli army withdraws from the positions it now holds, well-armed units of the Lebanese army and the expanded UNIFIL force mandated by the ceasefire resolution move immediately in on its heels and demand that Hezbollah forces remaining in the area vacate it and move north of the Litani River. Hezbollah — licking its wounds, loathe militarily to confront an international force, and not wishing to be accused of plunging Lebanon into more chaos — complies. Syria and Iran have no choice but to make their peace with this. The Lebanese-Israeli border is pacified and long-term quiet reigns there for the first time in decades. Hezbollah, no longer able to claim that it needs to muster a fighting force to defend Lebanon against Israel, lays down its arms and begins to function as a purely political party.

In a word, Israel has won the war.

But there is a worst-case scenario, too. It goes like this:

Israel withdraws rapidly from southern Lebanon, leaving behind hundreds of Hezbollah soldiers who have survived the fighting. Hundreds or thousands more join them immediately, many pretending to be returning refugees from the towns and villages of southern Lebanon attacked by Israel. Syria and Iran quickly re-arm them. By the time the Lebanese army and the UNIFIL contingents arrive on the scene in any force, Hezbollah is entrenched again all across the Lebanese south and refuses to leave. Only massive force can dislodge it, and neither the Lebanese army or UNIFIL is willing to contemplate its use.The situation has returned to what it was before the fighting broke out, and Hezbollah — still in command of the south and its reputation at a new height in the Arab world — is as dangerous to Israel as ever.

Which is to say: Israel has lost the war.

These are of course the two extreme possibilities. There are all kinds of intermediate variations.These too would involve a complex interplay of forces, of which Israel is only one — and far from the most dominant.

And yet if Israel does not play its cards well, the outcome is far less likely to be a good one. And the initial signs are not encouraging.Before the last shot had even been fired, the Israeli army was telling journalists that it intended to withdraw from southern Lebanon as quickly as possible, turning over responsibility to whatever international representative “arrived with a flag” to plant there. And to make matters worse, Foreign Minister of Israel, Tsipi Livni, has been telling other journalists that Israel will accept as part of the international force troops from both Malaysia and Indonesia, two hostile countries that do not recognize the Jewish state and have no diplomatic relations with it.

If this is what Israel intends to do, the war indeed has been lost.

One can well understand Israel’s anxiety not to get stuck with its troops on the ground in Lebanon forever, as happened after its 1982 invasion of that country, which it did not entirely leave until 18 years and hundreds of dead later, long after it should have.

But leaving too early can have, this time, even worse consequences. If Israel does not insist on pulling out of southern Lebanon slowly and carefully, and on first making sure that each town and sector that it leaves is under the firm control, and not just the theoretical jurisdiction, of the Lebanese army and UNIFIL, it will have accomplished nothing. Once more the dead and wounded will have been in vain.

And there are also, of course, good reasons for Israel’s seeking to improve relations with Muslim states like Malaysia and Indonesia. If either wants to send its badminton or water polo teams to Tel Aviv, they should be welcomed. But soldiers to Lebanon to guard Israel’s border against a radical Muslim terrorist organization? If this is the degree of seriousness with which Israel takes the international force, the latter may as well stay home.

Many criticisms have been made of the way Israel fought the war that has just — let’s hope permanently — ended. At least some of them are justified. But it is one thing to make mistakes on the battlefield or in the war room, when the bullets are flying and it is difficult to think clearly — especially when some of these mistakes may be caused by the praiseworthy desire to safeguard the lives of one’s own soldiers. It is another to make them in relative calm on the day after the war is over, thereby jeopardizing all its gains.

Security Council resolution 1701 is not perfect, but it is, from an Israeli perspective, a good one. If it is carried out in full, the 33-day war in Lebanon will have been worth fighting.But it can only be carried out if Israel makes it clear that it will not tolerate anything less. This is not the message that it is sending out in the first hours of the ceasefire.

Mr. Halkin is a contributing editor of The New York Sun.