Britain had spent decades tracking merchant ships and trade, with some assistance from Board of Trade data. The January 1940 Asama Maru incident illustrates the practical reach of that information system. British forces intercepted the Japanese liner near the Japanese coast and removed German merchant sailors trying to return home. This later historical example helps explain how a blockade in the 1939 counterfactual could target particular movements far from British waters.
Notable Point · 1:08:10-1:09:12
Asama Maru demonstrates the reach of British shipping intelligence
The later Asama Maru interception illustrates the detailed merchant-traffic knowledge behind Britain's proposed distant blockade.