‘So the king of the north will come, and throw up siege works, and take a well-fenced city. And the arms of the south will not withstand, nor the people of his choice, nor will there be any strength to withstand.'
Most see this as a reference to the capture of Sidon by Antiochus. The boy king Ptolemy V, who had succeeded his father, had sent one of his best generals to oppose him, but the Egyptian forces were defeated at the headwaters of the Jordan (near the Biblical Dan) and eventually surrendered at Sidon.
‘The arms of the south will not withstand, nor the people of his choice, nor will there be any strength to withstand.' The ‘arms' denote strength. Here the Egyptians were not strong enough. ‘The people of his choice' are probably his finest and best warriors selected to deal with the attack.