What did the americans threaten to do after germany sank a second american ship

By entering World War 1 on the Allied countries

battle of saint-mihiel, (12–16 september 1918), allied victory and the first u.s.-led offensive in world war i. the allied attack against the saint-mihiel salient provided the americans with an opportunity to use their forces on the western front en masse.

explanation:

a big wall across the mexican border so no drugs, terrorist, or illegal immigrants can come through : )