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The truth is that, while we Welsh like to believe that it was the mountains that beat the successive invaders, it was really the weather that comes with mountains. It was the rain that defeated every invader. Yes, simple rain.
— Grandfather ‧ The Englishman Who Went Up a Hill But Came Down a Mountain
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker gently deflates a proud national story. Welsh people like to credit the mountains for defeating invaders, but the claim here is that the real force was the wet weather mountains bring. The closing "Yes, simple rain" adds a dry, amused sense of anticlimax.
Context reviewed