According to Oxford Islamic Studies Online, dar al-harb is:
Territory of war, denoting the territories bordering on dar al-Islam (territory of Islam), whose leaders are called upon to convert to Islam. It refers to territory that does not have a treaty of nonaggression or peace with Muslims – those that do are called dar al-ahd or dar al-sulh.
Jurists trace the concept to Muhammad , whose messages to the Persian, Abyssinian, and Byzantine emperors demanded that they choose between conversion and war. When the leaders of dar al-harb accept Islam, the territory becomes part of dar al-Islam, where Islamic law prevails; conversely, according to the majority of jurists an Islamic territory taken by non-Muslims becomes dar al-harb when Islamic law is replaced. Like other classical legal concepts, dar al-harb has been affected by historical changes, and with the fragmentation of the Muslim world into numerous states, the concept has little significance today.
Now, I have to disagree with the last sentence of this defintion. There seems to be an agenda amongst some more fundamental Islamists in Europe that European territory should be brought within the realms of Islam – particularly, I believe, amongst Salafis -followers of the Salafist doctrine that is centered around the concept of looking back to a prior historical period in an effort to understand how the contemporary world should be ordered. They reject religious innovation or bid’ah, and support the implementation of sharia (Islamic law). The movement is often divided into three categories: the largest group are the purists (or quietists), who avoid politics; the second largest group are the activists, who get involved in politics; the smallest group are jihadists, who form a small minority. In legal matters, Salafis are divided between those who, in the name of independent legal judgement (ijtihad), reject strict adherence (taqlid) to the four Sunni schools of law (madhahib), and others who remain faithful to these.
Below are some videos that show how multiculturalism is a foreign concept in Europe when it comes to trying to integrate many Muslim migrants into traditional European societies. Europe, in its present state, is dar al-harb and Islamic fundametanlists living on its territory will always feel out of place or excluded because of their religious beliefs.