I believe the United Kingdom will end up going to war with a big foreign power sooner or later, whether it’s in 100 years from now or in 1 year from now, and I personally wouldn't feel safe knowing that our numbers have decreased.
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If for example a big foreign power invaded us, I don't believe that we would have enough troops to spread across the UK to help defend it and deploy our forces to fight abroad at the same time. If our numbers kept decreasing then I don't think that we would even be able to defeat some smaller countries. Our Air Force is also something that I believe lacks in fighter jets/pilots, if we don't have enough fighter pilots then our ground forces won't stand a chance because they wouldn't have enough air support.