Complex sentence: Amherst was summoned home, ostensibly so he could be consulted on future military plans in North America, and expected to be praised for his conquest of Canada but instead, once in London, was asked to account for the recent rebellion.
Split into simpler sentences: Amherst was summoned home, ostensibly so that he could be consulted on future military plans in North America, and expected to be praised for his conquest of Canada. However, instead, once in London, was asked to account for the recent rebellion.