The situation was little different from today. The aggressive, Muslim Suljuk Turks had taken most of present day Turkey and Northern Syria from the Byzantines in 1071, and later 1074. This included the ancient Christian cities of Aleppo and Ephesus. Massacres and forced conversions followed. The Pope and Western kings knew that if Constantinople fell (the West also felt the Byzantines, though Christian, were soft and corrupt), the whole East of Europe was exposed. The Turkish invasion of Europe actually DID happen in the 14th-17th centuries, so they were absolutely correct in that later point.
The Catholic Church was the only "transnational" figure of the day in Europe, so it fell the Church to point this out to, and rally, the various kings and fiefdoms of Europe.
Thanks for the history lesson, PGR88. It still boggles my mind to think that the president would use a National Day of Prayer to “enlighten” us. He really thinks that what we are seeing today has some moral equivalency in history. And he further uses a day of prayer to say this when he has told us all along that this has nothing to do with religion.