REHOVOT, Israel - From the time Israel came into being in 1948, it has depended on its army for protection from hostile neighbours. In purely arithmetic terms, this is a daunting task: Arabs outnumber Jews in the Middle East by a ratio of about 50:1. Taken together, the Arab states that could threaten Israel with ground forces -- Egypt, Lebanon, Jordan, Iraq and Syria -- field about 1,300,000 active troops. Israel's active army, with about 185,000 soldiers, is just one-seventh that size.

In times of war, however, Israel can count on another 445,000 reserve soldiers, men in their 20s, 30s and early-40s who once passed through Israel's regular army as conscripts. In case of an attack, Israel's strategy is to have its active troops hold off advancing Arab armies long enough for reserve units to organize and deploy. In peace time, reserve soldiers spend several weeks a year training and performing guard duties.

On March 29, the Israeli government called up 20,000 reserve soldiers for duty in the West Bank, where they fought side-by-side with professional units. Reserve soldiers saw heavy fighting in the Jenin refugee camp, where 13 of their number were killed in a single ambush on April 9.

"They needed a lot of troops for the West Bank operation," says Yiftah Shapir, a research associate at the Jaffee Center for Strategic Studies in Tel Aviv, and the co-author of The Middle East Military Balance, 2001-2002. "In many cases, you want elite infantry soldiers to do the job. But the army didn't have enough to go around. So they called up reserve troops."

No one knows for certain whether the use of reserve units in Jenin contributed to Israel's high casualty total. "It's impossible to know for sure," Mr. Shapir says. "I'm really not sure [professional soldiers] would have done any better, or taken fewer casualties. There was an ambush."

In fact, for the sort of operation conducted in the West Bank, reserve soldiers may be superior in some respects to the teenagers who staff the professional army.

"You need maturity when you're dealing with combat in civilian centres like Jenin. The army might prefer people who were out of the [regular] army for a few years -- soldiers who had families, who were better educated. Eighteen-year-old boys can sometimes be a lot more hot-headed than soldiers who are 30."

J.H. Kalman is a 35-year-old technical writer who lives in the quiet Tel Aviv satellite city of Rehovot. He works for a manufacturer of diagnostic equipment for printed circuit boards. On March 29, members of his reserve armoured brigade were called at home and told to report to unit headquarters near the port city of Ashdod. Four days later, Mr. Kalman's unit rolled into Jenin.

Mr. Kalman says he found the transformation to military life difficult. One day, he was laying out graphics for a technical manual. A few days later, he was operating the 105-mm cannon of a Patton M-60 tank, firing shells at targets in Jenin. His tank was stationed inside the Jenin camp for 24 hours at a stretch. The army ordered its hatch to remain closed throughout the operation.

"Packing up to go fight was one of the hardest things I've ever had to do," Mr. Kalman says. "You don't know what you're getting into. "

In 1985, Mr. Kalman served in Lebanon during his mandatory service with the regular army. Though his tank was under greater military threat in that theatre -- the Lebanese militias had anti-tank weapons -- he found the West Bank campaign more unnerving.

"In those days, being in the army was my job, so fighting in Lebanon did not seem like such a big deal. Now, I'm a civilian, and the idea of fighting frightens me. When I got the phone call [from the army], I hoped maybe we'd be deployed to the Jordan border or something for guard duty. Then I found out we had to go into a refugee camp."

Because the campaign was the first major Israeli military operation to take place in the Internet age, the use of reserves has created an unprecedented phenomenon: Many of the ordinary troops who returned to their white-collar jobs in late April used e-mail to describe their combat experiences during the incursion. Their narratives were forwarded around the world, and became part of the ongoing debate about Israel's actions in Jenin.

One 38-year-old reserve soldier, a father with three children, for instance, writes that: "When we entered the camp we were convinced that within a matter of hours, or at most a day, these terrorists would give themselves up.... Much to our surprise we discovered not a refugee camp but rather a terrorist base."

"We received heavy fire from many rooftops, windows, and even from the minaret of one of the mosques," he continues. "We conducted hours of gunfire to dismantle the enemy.... The sniper on top of the mosque hit and injured several of our soldiers. We could have taken him out by using a shoulder missile; however, that would destroy the mosque. Finally one of our snipers had a clear shot and took him out."