Britain's food reliance and the 70-day wheat clock drive the strategic threat at sea.

The submarine's invisibility changes naval warfare, turning cargo ships into vulnerable targets.

Cruiser rules fall away as war zones and unrestricted submarine warfare reshape tactics.

The Lusitania sinking tests public opinion and pulls the US toward intervention.

Convoys, escorts, and codebreaking bend the Atlantic war in Britain's favor.

The blockade's enduring lesson: invisible pressure can match, or exceed, visible power.