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In the event of war, the enemies of the US will have important information about everything.
Some classified documents may contain information that, when made public, would lead to greater harm than good. For instance: military secrets, intelligence operation details, embarrassing personal correspondence, security measures on critical infrastructure, enemy propaganda, etc.
This premise is nowhere near specific enough. For instance, one ought to know when they will declassify them. Or whether the time limit is different for military documents than civilian (hint: it likely should be). Or if they'd be redacted when they were declassified.