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Grvstu In what respect?
And you have not yet defined what you see as the "main terrorist threat" that is "obvious" to "we all"; nor where.
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The extremist threats to the
UK and European countries are from both Islamicists and far-Right political ideologists.
The extremists themselves are generally native residents of those countries; some groups, some individually (the "lone wolf" types).
The Islamic extremists tend to sympathise with ISIS' dream of its own version of the Third Reich; with somewhat similar attitudes to the original Nazis, Stalin's USSR and Pol Pot's Khmer Rouge.
The political ones tend to exploit fears raised by the ISIS types and by continuing, large flows of immigrants escaping wars, droughts and poverty in Africa and the Middle East. Some do admire Adolf Hitler's regime.
These have been known for a long time.
The warning from MI5 is of the Afghanistan situation encouraging such extremists in Western Europe.
Within Afghanistan, the Taliban may face problems from its rivals, ISIS, as well as domestic resistance; but if ISIS really causes trouble there I wonder if and how China will become involved. After all, the hard-line atheist Beijing government has announced its support for the hard-line theocratic Taliban. Just don't mention the tragic (Muslim) Uyghurs...
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America has her own internal threats from her own versions of both far-Left and far-Right, arising from deep social and political divisions and problems within American life. Nevertheless, the 20th anniversary of the World Trade Center atrocity by Al Quaeda, being commemorated in the UK as well as the USA, will warn us all that she need still be well aware of external threats.
Meanwhile over here, our thoughts are with you on this memorial day.