Complex sentence: On the eve of the struggle with Napoleon, Alexander, conscious of his unpopularity, conceived the idea of making Speransky his scape-goat, and so conciliating that Old Russian sentiment which would be the strongest support of the autocratic tsar against revolutionary France.
Split into simpler sentences: On the eve of the struggle with Napoleon, Alexander made Speransky his scape-goat. Alexander appeased Old Russian sentiment, the strongest support of the autocratic tsar against revolutionary France.