When Admiral Henning von Holtzendorff, the Chief of the Admiralty Staff of the Imperial German Navy, persuaded the German High Command to resume unrestricted submarine warfare from 1 February 1917 he admitted that this might bring the USA into the war. However, he argued that the U-boats would have sunk enough merchant ships to force the UK to surrender before the USA could mobilise a large army and transport it to Europe.
In fact the U-boats were unable to prevent US troops travelling to Europe. By the end of the war 2,079,880 US soldiers had reached Europe, 51 per cent in British ships, 46 per cent in US ones and most of the others in French ones, with a few travelling in Italian vessels. Most of the escorts, however, were from the USN: 83 per cent versus 14 per cent from the RN and 3 per cent from the French navy.
A significant number of the US troops travelled in one of 18 German owned liners that had been interned in US ports and requisitioned. They had a total tonnage of 304,720 tons and capacity of 68,600 men. The largest of them, the Vaterland (54,282 tons), renamed Leviathan, could carry 10,680 troops. She and three large British liners, the Mauretania (31,938 tons, 5,162 troops), Aquitania (45,467 tons, 6,090 troops) and Olympic (45,324 tons, 6,148 troops) were considered to be so fast and seaworthy that escorts could not keep up with them. They consequently made most of the passage on their own, being met by destroyers near their destination ports. They carried 135,467 out of 1,037,166 Canadian and US troops transported to the UK in 1918. The other troopships travelled as part of escorted convoys.
The largest loss of life of US soldiers to a U-boat came on 5 February 1918, when UB77 (Kapitänleutnant Wilhelm Meyer) sank the SS Tuscania (14,348 tons) off Islay, the southernmost island of the Inner Hebrides. The Tuscania, which was part of a British convoy, was carrying 2,000 US troops from Halifax, Nova Scotia to Europe. The loss of life was made worse because some lifeboats overturned as they were being lowered and because they then tried to make land rather than wait near the ship to be rescued.
There is some dispute about the number killed when the Tuscania sank. Lawrence Sondhaus’s recent German Submarine Warfare in World War I and the website U-boat.net both state that 166 soldiers and crew were killed. Archibald Hurd’s The Merchant Navy, part of the British Official History of the war, says 44 crew and about 100 soldiers. R. H. Gibson and Maurice Prendergast’s older work The German Submarine War 1914-18 says 44 crew and 166 soldiers, which is repeated in Paul Halpern and Robert Massie’s histories of the war at sea.
The only other US troopship sunk by a U-boat on her way to Europe was the armed merchant cruiser Moldavia, which was part of a US convoy when torpedoed in the English Channel on 23 May. Fifty-six US soldiers died. The largest loss of Europe bound US troops came on 6 October 1918 when the troopship Kashmir (8,985 tons), whose steering had jammed, accidentally rammed the troopship Otranto (12,124 tons), which ran aground off Islay with the loss of 369 soldiers and 69 crew.
Three more US troopships were sunk by U-boats, but on the way home when they were less well escorted: the Antilles (6,800 tons, 67 dead) on 17 October 1917, the President Lincoln (18,162 tons, 25 dead) on 31 May 1918 and the Covington (16,339 tons, 6 dead) on 1 July 1918.
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), pp. 435-6.
 Ibid., p. 436.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918 (London: Constable, 1931), p. 288.
 Ibid; Halpern, Naval, p. 436; A. S. Hurd, The Merchant Navy, 3 vols (London: HMSO, 1921). vol. iii, p. 285; R. K. Massie, Castles of Steel: Britain, Germany, and the Winning of the Great War at Sea (London: Jonathan Cape, 2004), p. 762; L. Sondhaus, German Submarine Warfare in World War I: The Onset of Total War at Sea (Boulder MD: Rowman & Littlefield, 2017). Kindle edition, loc. 3952 of 5745, Chapter 7.
 Halpern, Naval, p. 437.