Operation Sealion was the combined attack on the United Kingdom by the Greater German Reich and the Zheronians, having recently cemented their alliance.
Zheronian ships began landing in occupied France and Belgium, to the shock of many. The German soldiers were shocked at what appeared to be a godsend, and the civilians in the occupied territories were horrified.
Troop loadings continued throughout June, and the ships flew into orbit. The Zheronians generously provided technological upgrades to the German invasion force. The standard infantry was given advanced laser-based rifles, and the Panzer II was outfitted with a laser cannon specifically designed to pierce enemy armor. The Luftwaffe was reoutfitted with jet engines, made far more aerodynamic, and given laser cannons (to fighters) and antimatter-based bombs (to bombers).
The invasion began by the destruction of the cities of Portsmouth, Brighton, Dover, Bexhill, and Ramsgate by Zheronian orbital laser cannons, to prevent any interference with the chief attack. Zheronian ships, stocked with German troops, as well as Zheronian bat troops, warped over London, a city designated by Hitler as a city to spare. German soldiers were dropped into London, and with advanced technology, easily overpowered the British. Zheronian flying troops and spacecraft easily decimated the RAF.
The British Government tried to escape to Birmingham, but to no avail: the trucks were destroyed with lightning precision from space. Winston Churchill, Bernard Montgomery, and many other important ministers were killed.
With the world in shock, Britain saw little reason to continue to fight. The British forces asked for a ceasefire, and Germany agreed, negotating a peace in London. Oswald Mosely was appointed by Hitler personally to become Gauleiter of the newly created Reichsgau Grossbritannien, an integral part of the Reich.
With the Reich successful in annexing Britain, Hitler and the Zheronians put their efforts into a new plan: Operation Barbarossa.