Complex sentence: Mozart's reaction to the British victory was to declare to his father that he considered himself a "real Englishman"--an unusually political outburst from someone who was ordinarily fairly apolitical, perhaps reflecting antipathy towards the French.
Split into simpler sentences: Mozart's reaction to the British victory was to declare to his father that he considered himself a "real Englishman". This was an unusually political outburst from someone who was ordinarily very apolitical, perhaps reflecting antipathy towards the French.