Something unusual is happening along Israel's border with Lebanon: nothing.

The 49-mile stretch, one of the Mideast's most volatile areas, has been uncommonly quiet since the 2006 war between Israel and Hezbollah. Even as both sides continue to build up arms and make war plans, it's been one of the longest lulls in fighting since Israel's founding.

Not even a brief gunfire exchange last summer or the recent restructuring of Lebanon's government by Hezbollah have substantially raised border tension, as might have occurred a decade ago.

What's behind the relative calm? Many suggest that both sides are calculating that the price of the next conflict would be too high to bear, a miniature version of the so-called mutually assured destruction theory that some say helped prevent nuclear war between the United States and the Soviet Union.

"It's a little bit like the Cold War," said Mideast analyst Sahar Atrache of International Crisis Group in Beirut. "Two sides, a military buildup, but also awareness that a battle would be too costly for both."

On the Israeli side, military officials say Hezbollah has been deterred by Israel's demonstrated willingness to use its military superiority, last seen in the 2006 conflict, which killed 1,200 Lebanese and 160 Israelis. Israelis say memories of that war are the reason Hezbollah has halted the rocket attacks and cross-border raids that were once common in northern Israel.

More recently, however, Israel has found that it also faces a menacing deterrent in the form of Hezbollah's stockpile of an estimated 40,000 to 50,000 rockets, more than four times the number the group had in 2006. The arsenal, built with help from Syria and Iran, has a longer reach and better accuracy than ever before. Itis said to include ballistic missiles, possibly Scuds.

Israeli officials estimate that the Lebanese group's militia today can strike deep inside Israel, including heavily populated Tel Aviv, which was out of reach five years ago.

"It's a totally different situation," said retired Israeli army Maj. Gen. Jacob Amidror, now a defense analyst who is being considered to become Israel's next National Security Council chief. "Israel has never had such a threat to its homeland area. No question we will pay a higher price."

In the 34-day conflict in the summer of 2006, Hezbollah launched about 4,000 rockets, or an average of 120 a day, killing 44 civilians. Most rockets were homemade, short-range and caused no damage because they fell on vacant land. Nevertheless, large evacuations in northern Israel cost the economy about $4 billion.

Hezbollah has rearmed with Iranian-made long-range missiles capable of reaching nearly 200 miles, M-600 guided surface-to-surface missiles accurate to 500 yards, and — Israeli officials said last fall — Scud missiles supplied by Syria.

Syria denied the allegations, and U.S. and French officials have been unable to confirm the claim.

But U.S. officials in Syria said in a November 2009 cable released by WikiLeaks "that this buildup may portend a shift in the military balance between Israel and its northern nemesis" and that Hezbollah's capability to inflict physical and psychological harm on Israel had taken a "quantum leap" since the last war.

In a Feb. 26, 2010, cable, U.S. officials said they had information that Syria had trained Hezbollah on surface-to-air missile systems.

For Israel, this means much darker predictions for the next conflict. Israeli officials are bracing for up to 600 rockets a day and potentially hundreds of civilian casualties in a short period, something Israeli society has never experienced.

"When you think about what a clash now would do, it gives people pause," said a senior Foreign Ministry official speaking on condition of anonymity. "The stakes have been made much higher. But they will be higher for them too, if they go there."

Though Israeli government officials say they are not afraid to engage Hezbollah and are confident of their victory, they express growing alarm over the group's arsenal, which they say is one of the largest ever assembled by a non-state entity.

"What we now have is a balance of deterrence," said a senior military official, who also agreed to speak only if not identified.

Hezbollah leaders boast that their military buildup is the primary reason the border clashes have slowed, saying the deterrence they have created has forced Israel to take a less aggressive stance.

"Israelis always launched their wars with a safe domestic front," Hezbollah Secretary-General Sheik Hassan Nasrallah told the Lebanese National News Agency last year. "This situation is gone for good.

"Today we have inaugurated a new era in which we will be bombed and we will bomb, we will be killed and we will kill.... This is their current strategic weakness."

Yet even as Hezbollah has acquired dangerous new capabilities, Israel has confined itself to public criticism rather than military actions or cross-border invasions to disarm or weaken Hezbollah as it has done previously.

"The ability to hit Tel Aviv was a red line for Israel in the past, but now we are seeing the red line pushed a little farther," Atrache said. That might reflect Israel's fear about Hezbollah's ability to unleash thousands of rockets against its cities, she said.

Some see Iran as a factor in Hezbollah's more moderate approach along the Israeli border, saying Tehran is pushing Hezbollah to refrain from rocket attacks or military operations so that such retaliation can be saved for a later time, such as if Israel strikes Iranian nuclear facilities.

Israel has been using the recent calm on its Lebanese border to boost its defenses, particularly against rocket attacks. It hopes to soon deploy Iron Dome, a short-range rocket-interception system, and it completed a successful test last month of the Arrow system, designed to knock out ballistic missiles and expected to be operational by 2014.

Some experts say the lull won't last.

"The mutual deterrence has worked until now to prevent any casual escalation, but it won't last forever," said Daniel Kurtzer, former U.S. ambassador to Israel. "At some point red lines will be crossed, and if there's a situation in which either side believes its interests call for it, they'll take action."