Kaliningrad: How did Russia acquire this enclave?

If you look at a map of Russia today, there is an odd anomaly that doesn't seem to make sense to most of us born after the end of WWII. 

This is Kaliningrad an area of Russia separated from Russia proper by Lithuania. How?

Well, it is an interesting story in maps.

Kaliningrad was founded almost a century ago by the Teutonic Knights as Konigsberg in 1255. They effectively ruled the city for nearly 600 years. But now to relatively recent history.

Let's start at 1871, when the various duchies, monarchies, principalities, oligarchs and city states were united by Prussia to form Germany. It took 3 major wars, but the backwater of Germany was pushed into the forefront of history by the Prussian Emperor, Frederick III, and his military commander Otto Von Bismarck. The combined Germany included the traditional Prussia lands, including East Prussia, where Kaliningrad is now. 

Kaiser Wilhelm came to be Emperor of the unified Germany in 1888 - and dismissed Bismarck in 1890. The Kiser ruled until 1918 after Germany lost World War I.

After World War I, President Wilson and the Treaty of Versailles attempted to draw borders that capture the nationalities of its peoples. So Poland was carved out of the German and Russia Empires. But what to do with Koingsberg and East Prussia, where Germans lived? 

The decision (and treaty) kept East Prussia as part of Germany. The newly constituted Poland lay between the, connected by the Port of Danzig. The original plan had the city of Danzig a free port governed by the League of Nations. (Interesting side note, the city was renamed Gdansk, site of the anti-Communist riots that help drop the iron curtain.)

Germany always considered this situation temporary, and by 1936 the city was run locally by the Nazi Party. In 1939 Germany took the strip in the first invasion of WWII.

After Nazi Germany's defeat, the western powers created a Poland with new borders, and  created various spheres of influence in ex German territories. As you can see from the map below, Russia controlled the ex-East Prussia and renamed Koingsberg to Kaliningrad.

Most of the occupied lands were ultimately turned over to new countries, including the Soviet Sector of Germany proper into East Germany. But instead of given Kaliningrad up, the Soviet Union annexed half of East Prussian (the southern part going to Poland) and included it as part of Russia, not as part of one of the other Russian "states" - SSRs. It was due to its strategic placement as a warm water port.

When the Soviet Union dissolved and the states turned into countries, including Lithuania, the Russia SSR became the country of Russia, Kaliningrad stayed part of Russia under President Boris Yeltsin. Russia was becoming part of the new world, had a great relationship with the west and there was no reason to argue over Kaliningrad.

In 1990, Kaliningrad became a free economic zone as part of the Russian state. Now Kaliningrad borders NATO from both Poland and Lithuania. Thee are still war rumblings every now and then, usually about American missiles in Poland and Russian missiles in Kaliningrad, but the area is fairly peaceful.

So, now all is clear as mud.