As the later Roman Empire changed its form, several Germanic and Slavic tribes and the still-powerful regional noble families of the later Empire competed for power in different parts of Europe with the surviving portion of the Roman Empire commonly called the Byzantine Empire by modern Europeans. The spread of Christianity from the Mediterranean area and from Ireland and Scotland throughout Europe in tandem with the inefficiency of governmental structures meant that ecclesiastics became deeply involved in administering political institutions (for example the Holy Roman Empire and other medieval 'states') and formed the basis for a first European "identity": Christianity. An example of this identity at work is the period loosely identified as the Crusades, during which Popes, kings, and emperors tried to draw on the unity of all Christians to wage war on Islam, which was spreading along Europe's southern borders. Political unanimity in Europe was largely illusory, and the military support for most crusades was drawn from limited regions of Europe.
It is extremely difficult to decide when the Middle Ages ended, and in fact scholars assign different starting dates for the Renaissance in different parts of Europe. Most scholars who work in 15th century Italian history, for instance, consider themselves Renaissance or Early Modern historians, while anyone working in 15th century England is considered a medievalist. Others choose specific events, such as the Fall of Constantinople (1453) or the Discovery of America (1492) as its end.