Blücher entered the Oslofjord carrying troops and equipment for the seizure of the Norwegian capital, with surprise central to the plan. Heavy shells from Oscarsborg Fortress disrupted command and started fires among aircraft and army explosives, while open shore batteries damaged steering. German intelligence knew the locations of the main forts but had not identified Oscarsborg's concealed torpedo battery, whose two hits disabled boilers and turbines. Flooding and a 105 mm magazine explosion then ruptured damaged bulkheads and turned the tactical damage into a fatal capsize, with casualty estimates remaining disputed.
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How Oscarsborg Fortress sank Blücher
Blücher's rush toward Oslo shows how coastal guns, concealed torpedoes, embarked ammunition, and damaged machinery combined to destroy a new heavy cruiser.