"The Russians are probably not pleased their Kornets got into the hands of Hezbollah," said one former administration official familiar with Russian thinking.
Even without stepped-up involvement by Syria and Iran, military experts said, the ability of United Nations and Lebanese troops to track and halt arms shipments was likely to be limited.
The U.S. has identified nine Syrian border crossings that it thinks are large enough to ship medium- and long-range rockets.
Knowing likely shipment routes may not be enough to block them, analysts said, pointing to U.S. failure to prevent arms shipments to Iraqi insurgents from Iran and Syria -- or, in an earlier conflict, to the Viet Cong along the infamous Ho Chi Minh Trail from Laos and Cambodia during the Vietnam War.
"The bar is set very low, but it's still a very difficult bar to reach," said a U.S. defense official, referring to U.N. interdiction goals. "That border is still very porous."
Experts said Hezbollah retained significant weaponry even after the monthlong Israeli bombardment. Israeli intelligence officials think they destroyed a large number of Hezbollah's longer-range missiles, but do not know how many remain. The number of medium-range rockets in Hezbollah's possession also is unclear.
In addition to the 3,700 to 3,800 rockets fired by Hezbollah, the Israeli military said it destroyed about 1,600. Together, that would account for fewer than half of the rockets that Israeli and U.S. intelligence officials think Hezbollah had at the start of the conflict.
Israel said it had underestimated the number of Hezbollah fighters. The military said it killed about 500 Hezbollah guerrillas, a figure that the militia has not confirmed. Some of the fighters who were killed or captured were using sophisticated equipment, including sniper rifles and night-vision goggles.
"At most, if you take the most dramatic claim we heard, they probably got about 15% of Hezbollah strength, and that includes wounded as well as killed in the forward area, which is not a decisive type of battle," said Anthony Cordesman of the Center for Strategic and International Studies, a U.S. military analyst who recently returned from meetings with Israeli military and intelligence officials.
"If anything, you now have very large numbers of very experienced combat people who have spent more than six weeks in active engagement with the [Israeli military] and have, if not won, learned enough so they will be a far more serious problem in the future."
Spiegel reported from Washington and King from Jerusalem.