Assertion:  It is important to pay attention to context and situations when talking about security.
Reason:  After the Second World War, most of the European countries, particularly the powerful Western European countries, faced no serious threats from groups or communities living within those borders.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

While internal security was certainly a part of the concerns of governments historically, after the Second World War there was a context and situation in which internal security did not seem to matter as much as it had in the past. After 1945, the US and the Soviet Union appeared to be united and could expect peace within their borders. Most of the European countries, particularly the powerful Western European countries, faced no serious threats from groups or communities living within those borders. Therefore, these countries focused primarily on threats from outside their borders.