Complex sentence: Early in the ensuing battle, Downie, arguably the most experienced Officer in the British fleet, was crushed by a cannon aboard his flagship HMS Confiance and killed instantly, thus greatly increasing the odds of the American forces winning the engagement.
Split into simpler sentences: Early in the ensuing battle, Downie, arguably the most experienced Officer in the British fleet, was crushed by a cannon aboard his flagship HMS Confiance and killed instantly. Downie's death thus greatly increasing the odds of the American forces winning the engagement.