Israel that Egypt could retaliate if it bombed targets deep within Egyptian territory. Under cover of the initial artillery barrage, the Egyptian assault force of 32,000 infantry began crossing the canal in twelve waves at five separate crossing areas, from 14:05 to 17:30, in what became known as The Crossing. The Egyptians prevented Israeli forces from reinforcing the Bar Lev Line and proceeded to attack the Israeli fortifications. Meanwhile, engineers crossed over to breach the sand wall. The Israeli Air Force conducted air interdiction operations to try to prevent the bridges from being erected, but took losses from Egyptian SAM batteries. The air attacks were ineffective overall, as the sectional design of the bridges enabled quick repairs when hit. Despite fierce resistance, the Israeli reserve brigade garrisoning the Bar-Lev forts was overwhelmed. According to Shazly, within six hours, fifteen strongpoints had been captured as Egyptian forces advanced several kilometres into the Sinai. Shazly's account was disputed by Kenneth Pollack, who noted that for the most part, the forts only fell to repeated assaults by superior forces or prolonged sieges over many days. The northernmost fortification of the Bar Lev Line, code-named 'Fort Budapest', withstood repeated assaults and remained in Israeli hands throughout the war. Once the bridges were laid, additional infantry with the remaining portable and recoilless anti-tank weapons began to cross the canal, while the first Egyptian tanks started to cross at 20:30. The Egyptians also attempted to land several heli-borne commando units in various areas in the Sinai to hamper the arrival of Israeli reserves. This attempt met with disaster as the Israelis shot down up to 20 helicopters, inflicting heavy casualties. Israeli Major General (res.) Chaim Herzog placed Egyptian helicopter losses at 14. Other sources claim that "several" helicopters were downed with "total loss of life" and that the few commandos that did filter through were ineffectual and presented nothing more than a "nuisance". Kenneth Pollack asserted that despite their heavy losses, the Egyptian commandos fought exceptionally hard and created considerable panic, prompting the Israelis to take precautions that hindered their ability to concentrate on stopping the assault across the canal. Egyptian forces advanced approximately into the Sinai Desert with two armies (both corps-sized by western standards, included the 2nd Infantry Division in the northern Second Army). By the following morning, some 850 tanks had crossed the canal. In his account of the war, Saad El Shazly