How is unrestricted submarine warfare a threat to neutrality?
1 Answer
Observing the Maritime Laws of War requires a warship to halt and inspect merchant vessels before taking action; submarines are very vulnerable if they do this.
Explanation:
Most of a submarine's protection comes from remaining submerged; a surfaced submarine gives away its position and is very vulnerable to gun fire. They are also small and cramped vessels. Using a submarine to board and inspect merchant shipping during war is almost impossible. Accordingly, submarines usually don't bother.
In both World Wars, belligerents placed each other under blockade and used submarines as primary instruments of enforcing this.
Neutral ships are allowed to carry some cargoes into the ports of belligerent nations, and to conduct normal trade through war zones, but it is very hard to be accurately identified through a periscope lens and submariners are trained to be hunters. So... 'Torpedo los!'
In WW-1, American neutrality was severely challenged by U-Boat sinkings, although these alone were not enough to bring America into the war.
In WW-2, the U-Boats were notoriously careless, particularly off South America and a spate of sinkings in 1942 enraged Brazil and brought them into the war on the Allied side (Brazil made some significant contributions to the Allied War effort). Sinkings of coastal vessels, ferries, and major fishing vessels also drew Mexico to the Allied cause.
The U-Boats weren't alone in this. The Royal Navy torpedoed several Swedish ships. US submarines even sank American ships 'temporarliy' transferred to Soviet registration to carry Lend-Lease products to the USSR --the Russians were 'neutral' in the Pacific War until August 1945 and these ships had to pass through Japanese waters.
