How concerned should Americans be about homegrown terrorism in the U.S.? In the face of another plot by British Islamists, it's worth keeping in mind that America's Muslim community is strikingly different from those in Britain and the rest of Europe.
The first difference is in relative numbers. The most authoritative estimate of the number of Muslims in the U.S. is between 2 million and 3 million--less than 1% of the total population. In France, Muslims constitute about 8% to 9%; in the Netherlands, about 5.6%; in Germany, 3.6%; and in Britain, just under 3%.
More important, Muslims in Europe are concentrated...