Moving to destroy Hizbullah outposts across its northern border, Israeli troops engaged in fierce battles with the Shiite militia inside Lebanon for the first time in the week-old war.
Armored troop carriers and bulldozers Wednesday were ambushed by Hizbullah fighters, giving way to a clash that left at least two Israeli soldiers dead.
Hizbullah's abduction of two soldiers last week spurred Israel to launch a massive strike to destroy Hizbullah's military capabilities and isolate it by hitting Lebanese infrastructure.
With tacit support from the US, Israel has operated with a relatively free hand, leaving hundreds of Lebanese civilians dead. But that window of opportunity will close as the US ramps up cease-fire efforts and as images of ruin in Lebanon begin to dominate the international agenda.
"The clock is ticking, and the objective is to weaken Hizbullah to the greatest extent possible," says Yossi Alpher, the editor of the online Middle East affairs journal Bitterlemons.org.
"When the images on television of the bombing of Beirut, of Lebanese victims of destruction, and of infrastructure reaches a point where it pressures the US, [British Prime Minister] Tony Blair, and European leaders, then the process of winding down will begin. The pressure is starting now," he says.
As Israeli Foreign Minister Tzipi Livni held talks with European envoy Javier Solana, US Secretary of State Condoleezza Rice is poised to visit the region in the next week. Israeli observers speculate that gives the military only a matter of several more days to operate.
The Bush administration has refused to yield to international calls to press Israel for a prompt end to its campaign against Hizbullah.
Instead, Ms. Rice is trying to drum up support for what she called a cease-fire of "lasting value" – one that would have the Lebanese Army take over the south, where Hizbullah guerrillas have conducted a cross-border war against Israel for years.
With the limited time frame for Israel's offensive, analysts say that Israel is hoping to ultimately turn to diplomacy to broker a cease-fire that would include a return of the captured soldiers as well as the deployment of either the Lebanese military or a multinational force in the border region.
But it's unclear what type of international mechanism will be used to restrain Hizbullah after Israel's withdrawal.
"The most important thing is for Israel to explain to America, Europe, and the Arab world, why it is in their interest to have long-range missiles outside of Lebanon," says Gidi Grinstein, president of the Reut Institute, a think tank in Tel Aviv. Mr. Grinstein was part of the Israeli delegation to the 2000 Israeli-Palestinian peace talks in Camp David.
"This weapon is a tool by Hizbullah to undermine any political progress by Israel and the Palestinians. It's strategic nature isn't only its military impact on Israel, it's a tool in the hands of the Iranians and Hizbullah to have a finger on the button," he says.
After relying primarily on air power to search out rocket launchers and missiles used by the Lebanese guerrillas to attack cities and towns across northern Israel, Israel's military must now decide whether and when it will make a major push against Hizbullah on the ground, analysts say.
That's because its uncertain whether Israel can inflict a decisive enough blow from the air against Hizbullah's rocket launching operations.
The casualties from what appeared to be a limited foray Wednesday underline how the ground war in Lebanon is likely to be a much more difficult enterprise for the Israeli military after Hizbullah spent the six years since Israel's withdrawal preparing for such a scenario.
Israel said it wanted to destroy a Hizbullah position used to shell Kibbutz Avivim, a farming cooperative adjacent to the border. Israeli leaders have said a major goal of the operation has been to push the militia far away from the border.
And yet, Israel's ground operations aren't likely to resemble its sweeping 1982 invasion of Lebanon, but rather focused fighting that includes an exit strategy, Mr. Alpher adds.
"We're dealing with a guerrilla force, not a conventional force. We don't want to be occupiers and go from house to house. The objective is to avoid getting stuck there," he says.
Still hanging in the balance is another goal of the strikes on Lebanon: rebuilding a doctrine of military deterrence that has been eroded by six years of Hizbullah harassment in the north and rocket attacks by Palestinian militants in the Gaza Strip.
After a week of air, sea, and artillery bombardment of Lebanon that was free of Israeli casualties, an ill-fated ground operation would allow Hizbullah to burnish its image as a credible match for the vaunted Israeli military.
"Israel is perceived as country that can quite easily be defeated by means of attrition," says Avraham Diskin, professor of political science at Hebrew University in Jerusalem.
"Israel is not determined this time and if it fails to reach the breaking of the will of the other side, it will cost more lives and we will be dragged into another round in a few months or a couple of years from now."
Wednesday, airstrikes on Lebanon killed 57 civilians and one Hizbullah fighter, while the Israeli army said that two soldiers were killed and nine wounded when troops crossed the border to raid Hizbullah posts. Meanwhile, Hizbullah rockets killed two children and an adult in Nazareth, medics said. More Hizbullah rockets hit Haifa.
Israel said Tuesday it was ready to fight the guerrillas for several more weeks, raising doubts about international efforts to broker an immediate cease-fire. Israel also said its airstrikes had destroyed "about 50 percent" of Hizbullah's arsenal.
"It will take us time to destroy what is left," Brig. Gen. Alon Friedman, a senior army commander, told Israeli Army Radio.
Also Wednesday Israeli troops killed nine Palestinians in clashes in the Gaza Strip and the West Bank.
• Correspondent Rafael D. Frankel from Jerusalem and wire material contributed to this report.