Two penetrations of Islam into Europe

The first one happened in the 8th Century when the Arabs conquered Iberian Peninsula in 711 AD from the hands of the Visigoths.

The other one happened when Ottomans penetrated into Balkan Peninsula in the 14th and 15th Century.

Explanation:

After rise of Islam in 7th Century, the Arabs started spreading Islam into different areas. While doing so, they reached the Iberian Peninsula, and destroyed the state of Vandals. They even tried to conquer Frankish Kingdom, but failed to do so, after they lost the battle of Poitiers in 732.

In the 15th Century, a new country started its rise in the area of Asia Minor. That was the Ottoman Empire. They conquered most of the territories that were once part of Byzantine Empire, including the capital Constantinople. That is how Islam was spread in Eastern Europe.