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Why do historians debate whether Frederick II’s invasion of Saxony was a preventive war?

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Seven Years' War: Battle of Zorndorf — Seven Years’ War Frederick II leading his Prussian troops against Russians at the Battle of Zorndorf during the Seven Years' War, August 25, 1758.

Historians debate whether Frederick II’s 1756 invasion of Saxony was a preventive war because it raises significant questions regarding the morality and justification of preventive military action.

The debate centers on two conflicting perspectives:

The Argument for Necessity: By 1756, Frederick was facing a hostile coalition involving Austria, Russia, and France. He was genuinely threatened by an emerging anti-Prussian alliance, and his enemies—particularly the Russian empress Elizabeth—intended to destroy Prussia’s international status. From this viewpoint, Frederick’s decision to strike first was a rational attempt to forestall an imminent attack that he believed would lead to the destruction of his kingdom.1
The Argument of Aggression: Because Frederick took the offensive and initiated the conflict, his actions are often scrutinized as an act of aggression. Critics argue that regardless of the threats he faced, his choice to invade a neutral territory like Saxony unleashed a massive, destructive military struggle across Europe.1

Ultimately, while historians acknowledge that Frederick was under severe pressure, the act remains the most debated event of his reign due to the ethical complexities of launching a war to preempt a perceived future threat.1