Islam, not just radical Islam, is a threat to America and, indeed, to the entire world. The reason this is so is that, at worst, Islam is a murderous religion and, at best, an ambiguous (uncertain) religion.
In its murderous form, millions of people have been tortured and murdered. In its ambiguous form, millions of Muslims have stood by and watched as their fellow citizens were tortured and murdered.
For more information, read We Explain Islam (The Religion Of Muslims).