堪らなく
in a dreadful manner
恐ろしい方法で
as he looks at the mess he has left behind he must wonder how the Brits so often managed to succeed in the kind of situation where he has so dismally failed
彼が残した散乱を見て、英国人が、彼が失敗した悲惨な状況においてどのように成功に導いたのかを不思議に思うはずだ