Somehow Hitler becomes Chancellor of the German Empire after Germany wins World War I in 1918. Maybe the Great Depression is horrible in the German Empire, and it becomes popular to blame "Anglo-Jewish capitalists" for Germany's problems. Many Germans like TTL's Hitler (who is much more anti-British than OTL) want to dismantle the British Empire for good. Austria-Hungary has collapsed, but German efforts have strengthened the Kingdom of Hungary. Germany has annexed Austria, and has Bohemia as a puppet state. The Ottoman Empire has reformed and struck oil in the Middle East, transporting large quantities to Germany. The United States, isolationist in WWI, is likely to remain isolationist.
After he becomes Chancellor in 1932, Hitler and others promote the idea of military re-armament as a solution to Germany's economic problems. The Imperial German Navy is strengthened considerably, as is the Luftstreitkräfte. The Army isn't as big of a priority since Germany assumes that her allies in Eastern Europe will pick up the slack and Germany lacks any real rival on the continent, since even with British reinforcements, German soldiers will be parading in Paris before long, and the Russian Republic in the east is a complete mess who has to go through all the German puppet states.
So when the war breaks out in 1936, German forces defeat the Franco-British forces, with German commanders using blitzkrieg tactics to make up for the issues brought about by lesser funding of the army. Germans intercept the Royal Navy as they transport soldiers to the continent, sinking numerous capital ships and others at minimal cost. By autumn 1937, France has surrendered, and the majority of German actions are against Italy (guarding the passes alongside the Hungarians) and against France's Russian allies in the east. In the West, German warships and submarines interdict commerce to Britain, and Germany demands the surrender of Britain.
Fearing the loss of the British Empire to the faction advocating surrender, a military coup occurs in Britain and places a hardline faction in control of the country, demanding no surrender. Many agonising months go by, with the British war effort focused on countering German commerce (to little effect) and spreading propaganda internationally in an effort to get the Americans on their side (also to no effect, as the Americans don't feel like intervening in a war which doesn't effect them, although the US Navy sees a major buildup in the Atlantic just in case). Several major air raids occur on British shipyards, sinking several Royal Navy warships. The British break the chemical weapons taboo in early 1938 with chemical attacks on German bases in France and the Low Countries, prompting an immediate response in chemical attacks on London and other major British cities, which are increasingly bombed by the day as the Royal Air Force runs low on aircraft, equipment, and especially fuel.
In autumn 1938, forces loyal to Germany overthrow the Irish government, committed to neutrality because of their fear of British invasion. German paratroopers storm Belfast and reclaim Northern Ireland for the Free State alongside the Irish Republican Army (swollen with volunteers due to Britain's increasingly indiscriminate chemical weapons usage). An attempted British counterattack in Ireland is defeated, leading to huge losses and most importantly, another portion of the Royal Navy sunk.
The winter of '38/'39 sees invasion preparations by the Germans, as they plan to attack the British using bases in Ireland and along the Channel. In April 1939, Germans storm the Isle of Man, a "test run" for their invasion strategies--the attack is a resounding success. Finally, on June 6, 1939, Germans hit the beaches in the northwest and southeast of England, overcoming strong British defense. Months of harsh fighting occur as the British unleash everything in their arsenal--chemical weapons, innovative artillery and rockets, and even anthrax--to slow the German advance. But defeat is imminent for the Brits. London is encircled and cut off by autumn of 1939, and after months of brutal fighting and atrocities, German soldiers raise the flag of the German Empire over Westminster in early December.
On December 25, German Chancellor Adolf Hitler makes a Christmas day visit to the soldiers in London. And on that day as well, mutinies break out across Great Britain. On December 31, 1939, the British unconditionally surrender to the German Empire.
There, I tried.