United Kingdom's safety is determined by Belgian ground, France or Germany conquering Belgium/Holland is a lethal threat to UK.
Also, this is why UK could not stay neutral in both wars. Nobody can live with a knife next to throat every day.
In fact, had the Germans not invaded Belgium and Luxembourg to circumvent the main French fortification line, the UK would most probably have stayed out of the conflict, at least for a while (Germany was still seen as the attacker and offender and thus in a more negative light, which both French and British propaganda during the War milked to death). But that while might have been long enough to beat the French decisively.
It would have been a much different world today if that would have happened. But that one decision to go by the Schlieffen plan and thus draw the British into the conflict has changed the world in so many ways - mostly bad ones for everybody involved.
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