Germany and Britain
Britain dominated the North Sea and the Atlantic Ocean with their vast navy, however, Germany wanted to be in charge of the seas, and they wanted the biggest navy. Due to this a naval race is started. The HMS Dreadnought was first built by Britain; but Germany soon copied.
Germany and France
Germany captured Alsace Lorraine from the French, but the French wanted it back, and they wanted revenge.
Germany and Serbia
Germany was allies with the Austro- Hungarians but there was tension between Austro-Hungary and Serbia, meaning that Germany had tension with Serbia.