Saying they'd know America was a major power was a big if - keep in mind that before World War I, America had largely stayed out of European affairs. Sure, they sailed ships to fight the Barbary Pirates early in its history, and sure they fought the decaying Spanish Empire in Cuba and the Philippines in the Spanish-American War, but until America's actual entry into World War I, it was a second-rate power at best.
Consider manpower in the US military in 1916:
Now compare that to Germany which had 4.5 million active + reserve personnel in 1914 on the eve of war.
Even among naval forces, where the US had invested considerably in the decade leading up to WWI, the US with 245 warships including 36 battleships of which only 14 were modern dreadnought types in 1916 was dwarfed by the Royal Navy of 1914 which had 18 modern dreadnought designs with 6 under construction.
The US also lacked the long military tradition of the Germans, or experience in warfare that nations like the British and French had.
It's easy today to wonder what nations were thinking when they pushed the US into war in WW1 or WW2, but the perception of America as a power today was far from certain in 1917 or 1941.