Just what did the crusaders fight against? If not to regain Christian control of the area. Islam was a late comer to the area.
On a political note, the Pope parlayed the Moslem occupation of the Holy Lands into an effort to unite the fragments of Christendom, and, as a consequence, to remove, or at least reduce, the influence of Byzantium and its Eastern Orthodox version of Christianity.
Kathy -- your knowledge of history and geography is abysmal -- Christianity spread throughout the ROMAN Empire -- in Judea, in Egypta, in Syria and also in parts of the Parthian Empire -- in Mesopotamia, in Persia proper. The southern part of the ARabian peninsula was part of the Ethiopian Empire while Mecca, Medina etc, the eastern and northern part was Persian (northern Oman, Bahrain, the UAE etc.), the middle of the Arabian peninsula was not claimed by anyone as it was more or less worthless to them. Christianity did NOT spread there -- though Gnostic and Arian thoughts were widespread there (they are both Christian heresies -- Islam too is a Christian heresy)
Christians lost control over Judea, Syria and Egypt -- the Crusaders went to regain control of those lands.