So I know there have probably been a myriad of threads and posts on this topic, but after looking into how Unrestricted Submarine Warfare (USW) was actually used in the war, I had an idea that would make it both historically accurate and viable for Germany to attempt.
In the war, the Germans declared USW as a gamble. They knew they were hurting British trade and discouraging neutral nations (read: the US) from trading with them. Their chief concern was of course the fact that the US might be brought into the war, but to some senior members of the German command, this was worth the risk if it hurt Britain sufficiently.
I think this risk should be represented in the rule. Here’s my revised text:
Unrestricted Submarine Warfare
The German player can declare that he or she will begin unrestricted submarine warfare at any time during the game. Once
this is declared, Germany will conduct submarine attacks against both the United States’ and the British Empire’s shipping
routes during their respective Collect Income phases. On each of these powers’ turns, the Germany player will roll 1 die for
each German submarine in sea zones 2, 7, and/or 8. For each roll of 1-5, the attacked power will deduct this number of IPC’s from the income it collects during this phase. If a 6 is rolled (on either the U.S.'s or Britain’s turn), the United States will immediately enter the war on the side of the Allies.
I think this change makes USW tempting enough for the Germans to want to try (it is certainly NOT tempting now) and gives a realistic impression of the risk associated with the move.