It is 1920. The Germans have defeated the US fleet and landed an army in New England. Through the rest of book 1 the US army puts up a heroic and very shrewd fight, but still gets clobbered and chased out of New England. The army survives in spite of being outnumbered and outgunned. The basic sequence is: Germans clobber us. The Americans commandeer all cars in the area and bug out after moving all the industrial machinery that can be moved and destroying the rest. They set up a new set of defenses, then wait until the Germans catch up. Sequence then repeats.
Books 2 & 3 apparently have the Germans pushing east as far as Pittsburgh, with the Americans building up and stiffening their resistance.
Book 4: The Germans finally get theirs. We've been secretly building entire new armies with the vast resources of the continent, while delaying the German advance and husbanding our resources for the decisive moment. We've also been building a huge fleet of over 4000 transport airplanes, and mass producing submarines at the rate of 30 per month.
Finally, in 1921, the decisive moment comes. Our submarines sink a huge German transport fleet, killing 70,000 German soldiers. While the Germans are trying to re-establish command of the seas, we airlift entire army corps, including their horsed cavalry units, over German lines--80,000+ men in one night. The cream of the German army is trapped. The Germans scrape together a force to break through from the east. We wait until they're committed, then airlift another couple of army corps to cut the rescuers off. By this time the American army has 4 million trained and equipped men. The Germans have a million men on the continent. By this time the American army has a decisive edge in artillery firepower and completely controls the skies. The Germans are forced to negotiate a withdrawal from the continent.
Overall: Some interesting ideas. Some really strange ones too. German tanks even make a brief
appearance in the last book--pretty good since the copyright date is 1916 and the first actual use of tanks was
in September of that year.
The broad outline of the war is so much like what actually happened between Germany and Russia 25 or so years later in World War II that it's almost uncanny. The Germans win battle after battle but the opposition moves industry out of their reach, builds up overwhelming superiority in manpower and strategic mobility, then cuts off the cream of the German army. Sounds a lot like Eastern Front World War II up through Stalingrad. Here's a brief excerpt to give you some of the flavor.
From: H. Irving Hancock - The Invasion of the United States: Uncle Sam's Boys at the Capture of Boston (Henry Altemus Company, 1916)
"For these were feverish, yet carefully controlled, preparations. On this spring day in
the year 1920 a huge capable German battleship fleet was steaming toward some point, between Portland and New York,
on the North Atlantic Coast. Back of this fleet was another flotilla of transports carrying a German army numbering,
according to different estimates, between a hundred and twenty-five thousand and two hundred thousand veteran German
"Yes, the United States was at war with a first class European power..."
[U.S. Army preparations for a fight around Boston involved 38,000 troops, many of them poorly trained. Preparations also included a provision for mobility]
"In case it should prove that the troops were not needed there, but were urgently needed elsewhere, a line of six thousand automobiles waited a few miles back from the coast. These cars would snatch up the army and hurl it pell-mell to the real point of attack that might develop."