Perdiccas marched to attack Ptolemy in Egypt but when he reached the most easterly tributary of the Nile near Pelusium, he discovered that the opposite side was garrisoned. In response, he marched upstream to find a suitable point to cross, soon coming across a ford which led to the cities of Tanis and Avaris on the other side of the Nile. However a fort defended by an Egyptian force known as the 'Camel's Rampart' inhibited his advance. Perdiccas then ordered his war elephants and Silver Shield infantry to attack. A larger army under Ptolemy arrived, denying Perdiccas an easy victory. Despite this disadvantage, the attack proceeded but Perdiccas was overwhelmed and forced to retreat and search for another crossing. He came across one near Memphis but when he attempted the crossing the water became increasingly deeper and many of his men drowned.