(Note: I do not own or claim to own any of the work produced by Harry Turtledove.)
This TL actually diverges from Turtledove's 'Atlantis' timeline, whose original POD is somewhere 85,000,000 years in the past, where the east coast of OTL North America breaks off and forms it's own mini-continent (much akin to OTL's New Zealand). This being said, this scenario diverges during the French and Spanish War of Turtledove's TL. In 'Opening Atlantis', the British successfully defeat the French and Spanish in several theaters, including the Atlantean theater. This results in France ceding its territory in Atlantis to Britain. These territories would later rebel and form the 'United States of Atlantis,' purchasing Spanish territory in the south as well so that the USA controlled the entire mini-continent.
In this scenario, the French and Spanish War is fought to a stalemate. While Britain and their Prussian allies are technically the winners, less territory is exchanged. This results in France maintaining its hold on its Atlantean colony.
Flash forward to the mid-1900's. Europe has reached a tipping point and lines are being drawn. The Allies, (Britain, Germany, and Moscow) are facing down the Entente (France, Italy, and Austria-Hungary). For the second time in history, Europe has sparked another global conflict.
In Atlantis, the full effect of the war is not felt right away. French Atlantis begins building up their military forces to assist their motherland. Though the United States has a 'special relationship' with Britain and has much more Anglo-German leanings, the Consuls both assure the nation that they will do everything in their power to keep the US from war.
This all changed on May 7, 1942. French airships launched a surprise attack across the mouth of the Stour River (OTL Potomac River) into southern Freetown, the southernmost state in the Union that shared a name with its capital city. The airships bombed the Blackpool Naval Yard, where a sizable portion of the United States' Atlantic fleet was stationed. Three Hannover-class battleships, two Avalon-class battleships, two Destroyers, and thee cruisers, as well as numerous other smaller ships were damaged or sunk.
This attack shifted the attitude of the nation dramatically. The next day, Consuls Richard Andrews and Micah Praeger called an emergency Senate session to request an immediate declaration of war against France. They received it, after a moving speech by Praeger, with resounding applause.
The Atlantean Theater ignited with several skirmishes across the Erdre and Stour rivers, mostly ending in favor of France. As France pushed into Germany in Europe, French Atlantis surged north into the United States, quickly taking all of Freetown and much territory in the surrounding states. The 'high water mark' sat west of New Hastings, and north of Honker's Mill and Horsham. By this time, the Atlantean army had fully mobilized and stalled the French advance. From July to December of 1943, the front remained at the 'high water mark,' hampered by trenches, mountains, and foul weather.
Relief for the Atlanteans came in the winter of 1943-1944. General Josiah Radcliff, commander of the Army of the United States, launched a winter campaign into Freetown and successfully took back the city-proper and most of the state. While French troops were diverted away from the western front, Commanders Harem Knox and Carl Hand took back New Grinstead and crossed the Erdre to liberate the southwestern corner of the country. The French Atlanteans sent requests for more troops and supplies to their mother country, but France was finding itself slowly encircled on land and water by Britain and Germany and was unable to send more support.
With the United States-proper liberated, Radcliff launched an attack against the French capital of Cosquer, burning most of it completely to the ground. Most in the US saw this as retaliation for the May 7th attack. He then moved his troops into the interior of French Atlantis, aimed for the fort-city of Nouveau Redon. By this time, the other Allied Powers had enough troops and supplies to send fleets to aid the US. Sailing east from Terranova was an Ontarian fleet under Admiral John Wilme, and west from Britain was another fleet commanded by Winston Endicott. While Radcliff struck down the Loire River in the interior of the continent, Wilme assaulted the major French port of New Marseille and Endicott shelled St. Denis and the Banks.
As Britain and Germany closed in on Paris in Europe, Radcliff continued chasing the remnants of the French Atlantean government. He finally tracked them down in a small village on the banks of the Loire. Radcliff forced the Governor-General of French Atlantis to surrender in the early part of 1946.