WWI-era alternate-future novels featuring invasion of the USA

A few months ago, I was intrigued to see a book titled The Invasion of America on Project Gutenberg's RSS feed of newly-uploaded works. Published in 1915, it details the disastrous results of a sudden invasion on the US's east coast:
A "Great Coalition" invades through Rhode Island with a few hundred thousand troops and occupies New England and the New York City metropolitan area (plus Puerto Rico and Guantanamo Bay)--62,000 sq. mi. (160,000 sq. km.) and 30 million people. The US has only 31,000 mobile federal troops and 30,000 half-trained state militiamen in the contiguous states, and can do nothing but defend Panama with its anemic navy.

Eight months later, when the US finally has managed to amass and train a 1.5-million-man army, the Coalition merely ends the occupation and sails away, having siphoned something like $4 billion ($100 billion in 2016 dollars) out of the country without having to fight anything but the US's pathetic navy and half-empty coastal defenses.
It even has 173 endnotes, in which it cites sources and justifications for its claims!

Just this morning, I was surprised to see the similarly-titled The Conquest of America in the same RSS feed. Published in 1916, it details a German invasion:
1921-04-27: A German ship blows itself up in a suicide bombing of the Panama Canal, trapping the entire US fleet in the Pacific (in which it is performing exercises as a show of force against a belligerent Japan). Simultaneously, Germany declares war on the US. Only 50 airplanes and 25 submarines are available for the defense of the Atlantic coast. (The naval reserve of ten "predreadnoughts" is in Philadelphia, but there are no men available to crew the ships.)

1921-05-04: The Mayor of New York City, George McAneny, appoints a Committee of Public Safety, to remain in permanent session in Madison Square Garden.

1921-05-11: Due to public disorder, Leonard Wood, the general in command of the Eastern Army, places New York City under martial law.

1921-05-12: The German fleet arrives off New York City. On the US side, only ten airplanes and six submarines were assigned to the defense of that area, and can't put up much of a fight.

1921-05-14: 150,000 German troops land on Long Island. Only 30,000 mobile US troops are available for defense. (A German dreadnought is guarding the mouth of Delaware Bay, preventing the inferior US naval reserve from being deployed.)

1921-05-23: The German troops, led by Field Marshal Paul von Hindenburg, occupy the entirety of Long Island. General Wood blows up the bridges and floods the subways that connect Long Island to the rest of the city.

1921-05-25: The specie of the New York City banks (including the Federal Reserve Bank) is transferred to Chicago.

1921-05-26: In the middle of a speech by Bernard Ridder (son of Herman Ridder) emphasizing the loyalty of the city's German-Americans, 50,000 German spies take Manhattan Island in a surprise attack. The entire Committee of Public Safety, including such illustrious personages as Rockefeller, Morgan, Carnegie, and Vanderbilt, is taken hostage for $1 billion in gold ($10 billion in 2016 dollars). Wood's army (20,000 federal troops, 50,000 half-trained militiamen, and 5,000 raw recruits) retreats to Trenton, New Jersey.

1921-06-03: Another 150,000 German troops land in New York City.

1921-06-05: Two 125,000-man German armies, under von Hindenburg and Alexander von Kluck, march for Trenton and Boston, respectively, while 50,000 troops remain in New York City.

1921-06-12: New Haven, Connecticut, is captured with minimal resistance by von Kluck's army, which is accompanied by Ferdinand von Zeppelin and Crown Prince Friedrich Wilhelm. Twenty hostages, including ex-President William Taft, are taken to ensure the good conduct of the city.

1921-06-22: von Hindenburg wins the four-day Battle of Trenton, having taken 12,000 casualties to Wood's 15,000, and takes hostages to ensure Trenton's good conduct. Wood retreats to Philadelphia.

1921-06-27: Hartford, Connecticut, is captured without resistance, and hostages are taken.

1921-07-02: von Kluck's army enters Boston and takes hostages, having experienced fewer than twenty casualties from unorganized militias and firebombed the same number of villages in retaliation. Several thousand Bostonians riot, leading to a few hundred deaths among the citizens and the killing of two hostages.

1921-07-11: Admiral Frank Fletcher, having brought the US fleet from the Pacific to the Atlantic by way of Cape Horn, loses the Battle of the Caribbean Sea, between Guantanamo and Jamaica. The flagship super-dreadnought Pennsylvania is destroyed, along with the rest of the US fleet.

1921-08-17: Outnumbered two to one, Wood loses the Battle of Philadelphia and half of his army to von Hindenburg, and retreats to Delaware. A third German army, led by Anton von Mackensen, lands in Chesapeake Bay and captures Richmond, Virginia.

1921-08-18: The US sues for peace. A peace conference at Mount Vernon is planned for September.

1921-09-08: At the Mount Vernon Peace Conference, the US is represented by Major General Wood, ex-President Taft, and Nobel Peace Prize recipient Elihu Root, while Germany is represented by Field Marshal von Hindenburg, Overgeneral von Kluck, and Ambassador Johann von Bernstorff. The US offers $3 billion ($40 billion in 2016 dollars). Germany demands "all of New England, about one-third of New York and Pennsylvania (the southeastern portions), all of New Jersey and Delaware, nearly all of Virginia and North Carolina[,] and all of South Carolina and Georgia"--an expanse of territory approximately equal to the area of the German Empire, and to be called New Germany. After two weeks of negotiations, Germany has reduced its demand to New England and New York City, and the US is about to capitulate.

(Note: Past this point, the story seems to become a little more fantastical.)

1921-09-09: Thomas Edison, part of the Committee of Twenty-One--a group of patriotic businessmen who, since July, have been seeking to save the country--brings to the attention of the US government a method (newly invented by Lemuel Widding) of destroying the German fleet and preventing the enemy army from being supplied. Edison and Widding are kidnapped by a German spy before they can give the method to the US government--but, now apprised of the new situation, the US negotiators refuse to capitulate.

1921-10-03: The truce ends, and hostilities resume.

1921-10-14: At the Battle of the Susquehanna, Wood's 60,000 troops are arrayed against von Hindenburg's 150,000. However, when the Germans cross the Susquehanna River, it's revealed that Standard Oil rigged the US trenches with oil pipes. The trenches and the river are set ablaze, resulting in the deaths of 113,000 German troops and the capture of 7,000.

1921-10-15: The remaining 3,000 men of von Hindenburg's army, including von Hindenburg himself and Crown Prince Wilhelm, are caught by surprise and captured by Wood.

1921-11-05: In preparation for the Third Battle of Bull Run, 5,000 tons of liquid chlorine have been prepared, but only 500 small airplanes are available to drop it, and 300 more are required. In the nick of time, Alberto Santos-Dumont, Juan Bielovucic, and Horacio Anasagasti arrive with 400 planes! Despite "this splendid support given to America by her sister republics", however, Wood's army is forced to retreat into the Alleghenies.

1921-11-29: In seditious Milwaukee, Wisconsin, the unnamed President of the United States is asssassinated. His wife also is killed, in an attempt to shield him. This excites a fervor of patriotism across the country. German-Americans in particular, with Bernard Ridder as their public face, are emphatic in their loyalty.

1921-12-24: 600,000 Bostonians rise in revolt and take prisoner 4,000 German garrison troops.

1922-01-01: The America--a new, 600-foot-long "super-Zeppelin, with apparatus for steering small submarines by radio control", designed by Nikola Tesla and John Hammond Jr.--is destroyed in an attack on the German super-dreadnought flagship Bismarck in New York City. Tesla, who volunteered to captain the craft on its first mission, is killed.

1922-01-20: The German garrison of Richmond, Virginia, is slaughtered by a force of ten thousand insurgent "mountaineers". This setback, coming on the heels of the Boston revolt, inflicts serious damage on German prestige.
In the confusion, Thomas Edison is rescued from the city by the story's protagonist, who learned Edison's location from a repentant German spy some weeks ago. He reveals to the Committee of Twenty-One the "Widding-Edison torpedo"--"a sure way to make an ordinary Whitehead torpedo hit a battleship".

1922-01-24: A fourth German army of 150,000 men sets sail from Kiel.

1922-02-03: In the Atlantic Ocean, a group of US seaplanes armed with the new Widding-Edison torpedoes attacks the German troop-carrying fleet. It sinks six German ships (three dreadnoughts and three troop transports), with a loss of only nine seaplanes, before the fleet surrenders.

1922-02-22: Germany signs the Treaty of Pittsburg--a white peace, with no gains on either side.

1922-03-04: France and Russia attack Germany.
Contrary to what you might expect from the title, The Conquest of America actually has a happier ending than The Invasion of America does.

Does anyone have any opinions on these two books? Was this a particularly-popular subgenre, back in the 1910s?
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