Caesar's consideration of the Gallic request to enter Rome was not indecision, but a play for time. He was in Rome when news of the migration arrived, and he rushed to Transalpine Gaul, raising two legions and some auxiliaries along the way. He delivered his refusal to the Gauls, and then promptly returned to Italy to gather the legions he had raised on his previous trip and three veteran legions. Caesar now had between 24,000 and 30,000 legionary troops, and some quantity of auxiliaries, many of whom were themselves Gauls. He marched north to the Saône river, where he caught the Helvetii in the middle of crossing. Some three-quarters had crossed; he slaughtered those who had not. Caesar then crossed the river in one day using a pontoon bridge. He followed the Helvetii, but refused to engage in combat, waiting for ideal conditions. The Gauls attempted to negotiate, but Caesar's terms were draconian (likely on purpose, as he may have used it as another delaying tactic). Caesar's supplies ran thin on 20 June, forcing him to travel towards allied territory in Bibracte. While his army had easily crossed the Saône, his supply train had not. The Helvetii used this moment to attack Caesar's rearguard.