What event best illustrated geopolitics -an interest in taking for strategic purpose?

There is no single event that all experts in geopolitics agree is the best illustration of this term. The term is defined as " a combination of political and geographical factors relating to a state"

While no single event can be isolated as "best", there are a great many examples of where geopolitics has had major affects. Some to the point of wars that caused the deaths of millions of people.

Listed below is two examples of geopolitics, one is a simple example and few people may not be aware of it. The other example is one the major ones that were a plague on humanity.

Example A. Small time example:

During the US Civil War, it was known that one of the most despotic countries in the world was Russia. Yet it supported the US Federal government during the Civil War. The reason was based on geopolitics. Lacking sufficient warm water ports during the Winter, the USA allowed the Russian Navy to practice maneuvers off of the US East & West coasts. Russian support or recognition of the Confederacy had no advantage to it.

Example B. An example of grave importance:

Ever since Japan's modernization and becoming a recognized world power, its industrialized economy was dependent on other nations for the Natural Resources Japan had. It was a relatively small country and it lacked a bucketful of minerals, metal ores and petroleum products. The huge nation of China had these resources and there was petroleum in the Dutch East Indies.

The attack upon China in 1931, can be said to be an example of a geopolitical event of immense proportions.

There were other reasons for Imperial Japan's quest for war, but obtaining the natural resources it lacked by war was a large reason for Japan's military adventures beginning in 1931.