In light of the British strategist's comments about the threat posed by unassimilated and hostile immigrant groups in Europe, I thought it important to note why it matters despite the fact that they are not near a majority. Some would argue that it does not matter if they are not a majority because Europe is democratic. That would be true to an extent if the immigrant communities were not hostile and prone to violence. Even if they were not violent they would still be very influential in the democratic process since they would be seen as a rapidly growing unified voting bloc on the margins; just look at how much attention the Hispanics get here in the U.S and they are only around 6-7% of voters.
What is important when determining how much trouble such immigrant communities could cause is not overall population ratios but ratios of young men willing to fight. After all, it matters not that a country challenged by 2 million young men has 11 million people if 10 million are old women and only 1 million young men. For example, let’s say in 2020 the immigrant population of France has hit 20% of the total population (no one is certain of the exact demographic makeup of France but it is probably close). Since the immigrant birthrate is far higher than the native French rate (possibly three times as high but again no one knows for sure) they could represent 30-40% of the young men in France at that time though we will go with 35%. While Frenchmen would still be a majority there is a second aspect to consider, those willing to fight. This poll, from Britain admittedly, showed that only 19% of young men are willing to fight for their country regardless of the reason. Another 57% are willing to fight if they agree with the cause, though I am not certain how many would consider suppressing racial minorities, no matter how odious they are, to be a just cause. Assuming that the situation is similar in France and that half believe it is a just cause, then the 65% of young Frenchmen becomes about 31%. Also assuming that all immigrant men are willing to fight, they would outnumber the number of Frenchies willing to fight 35-31.
The sad reality is that in the end bullets count for more than votes. Even should the ratios favor the Europeans the immigrants could still cause quite a problem. Look at Northern Ireland where the British had to keep 20+ thousand soldiers for 40 years to deal with 400,000 Catholics. Can the French find 600,000 men to deal with 12 million Muslims? Does it even matter when the entire country is like Northern Ireland circa 1970? How would the Muslim countries react to their co-religionists revolt? If they decided to support them, could Europe in its wounded state resist effectively? After all, the Romans could keep out the barbarians even at their end (see the Battle of Chalons), it was the combination of civil war and barbarian revolts from immigrant (foederati) tribes that finished Rome. This is why, as the British strategist pointed out, the Europeans should be more concerned about their immigrant communities even though they will not be a majority anytime soon.