Francis Fukuyama writes (in Slate) that Europe's demographic/Islamist crisis is caused by its failure fully to integrate moderate immigrants into society.
Europe, unlike the USA, was more concerned to "tolerate" incoming communites as a whole, even if this meant overriding European values. Oppression of individuals within those communities (especially women and homosexuals) could thus continue, whilst deserving individuals who may have been westernised became radicalised instead.
Fukuyama thus urges European politicians to ignore calls from the "blood and soil" strand of the American right.
It's a valid point, as Europeans will not solve their "demographic crisis" simply by going to church and having more babies (even if politicians somehow found a way of making them do so, which seems unlikely). Then again, one should question the alarmism of those who fan and publicise this crisis, too. Some are actually hailing the crisis as a harbinger of the return of traditional patriarchy.
Europe vs. Radical Islam By Francis Fukuyama
See also: Fukuyama's End of Neoconservatism