When you pull samples and see that every tree had less growth x rings ago, you can determine aspects of weather.

Weather can have a huge impact on war. D-Day occured when it did because it was a clear day. Vikings spread from Scandinavia because there wasn't enough fertile land to go around. Napoleon in Russia. Similar theories exist for the bronze age collapse. The seas around Cape Horn meant that controlling the Cape of Good Hope was crucial for controlling global trade.