Invading a British ally was one. Declaring war on France, which is the gateway to Britain, was another. Rejecting Britain’s ultimatum with respect to Belgium was a de facto declaration of war on Britain, to boot.
Germany had no designs on the UK. She had to accept war with France as France was allied with Russia and had made no bones about her desire to get revenge for 1871. The Entente Cordial was not a formal alliance but more of a “fomal understanding”. The attack on Belgium was a major mistake, but England would have entered the war anyway to prevent the continued growth of the German fleet.