What fueled economic growth but also enflamed old rivalries and contributed to two world wars?

The economic and political practice known as New Imperialism. This was a major policy in the advanced economies of the late 19th century that sought to expand themselves through territorial expansion and control over strategic resources on the nations of "less civilized" populations. The qualifier 'New' is used to differ it from the first european wave of expansion on the Americas since the focus of this second expansion was on the African and Asian continents. Historical examples of this policy are the british domination of India, known as the British Raj, and the german trading outposts in China such as that in Tsingtao (modern day Qingdao).    

a countries leader should be motivated by the common good of the country.