。
A million people lost their lives in that war.
Literal
One-million-people [genitive-の] people [topic-は] that war [at-で] life [object-を] dropped.
命を落とす ('to drop one's life') is the standard literary/euphemistic expression for 'to die' in contexts of tragedy, war, or accident. More blunt alternatives are 死ぬ ('die') or 亡くなる ('pass away'). 100万人の人々 is a somewhat redundant phrasing ('the people of a million people') but emphasizes the human scale of the loss.