The goal of nearly every war in history is to strengthen one’s own territory and to expand it. The particularity of modern wars however is an attempt to limit state borders within one nation and language. This process turned out to be especially challenging as the actors attempted to do so in accordance with natural characteristics of border lands. Language groups and nations are seldom strictly divided; mostly they are fluid and ever changing by natural and historic fluctuations. After the First World War, new borders played a dual function: they were the result of the conflict and they became the source of the conflict. The revision of state borders was not an exclusive goal of Nazi Germany, while the desire to defend its own borders led the countries to build numerous defense lines. Incapability and immaturity of international community to resist the alteration of borders led to the outbreak of the biggest war in Europe, when new borders were drawn in accordance with historical aspirations of the Axis countries to create large-scale state entities, gain access to the natural resources or based on existence of minorities in a particular territory. The paper will illustrate the most important border disputes and defense lines in interwar Europe that had played a major part in the outbreak of the Second World War.