Shirer first takes note of Leopold in Chapter 9: The First Steps, as he describes the shifting nature of Germany's international relations after Hitler announced Germany's formal trashing of the Treaty of Versailles. Soon after, Leopold capitulated, against the wishes of the Belgian government, to Nazi Germany's invading forces.
Characteristically, Shirer can't help but make his own opinion of the man perfectly clear, though he tells us that it was also the judgment of the Belgian people themselves. As he explains, Leopold "was not recalled to the throne from Switzerland, where he took refuge at the war's end, until five years after it was over. When the call came […], his return provoked such a violent reaction among the populace that civil war threatened to break out. He soon abdicated in favor of his son" (4.21.81).