In 1839, the United Kingdom of Great Britain and Ireland, France, and Prussia (the largest predecessor of the German Empire) had signed the Treaty of London which guaranteed Belgium's sovereignty.
[1][2] However, the actual reasoning had at least as much to do with the British fright that lack of their help leading to a possible defeat of France could lead to German hegemony in Western Europe, with Christopher Clark pointing out that the British cabinet decided, on 29 July 1914, that being a signatory to the 1839 treaty guaranteeing Belgium's frontiers did not obligate it to oppose a German invasion of Belgium with military force.
[5] On 3 August 1914, Sir Edward Grey, the Foreign Secretary, announced to the House of Commons information he had received from the Belgian Legation in London.
It began:[11] His Majesty being mindful, now that a state of war exists between this Country and Germany, ...Winston Churchill later wrote: It was 11 o'clock at night – 12 by German time – when the ultimatum expired.
Under the roof from which Nelson had received his orders were gathered a small group of admirals and captains and a cluster of clerks, pencil in hand, waiting.
On this deep wave there broke the chimes of Big Ben; and, as the first stroke of the hour boomed out, a rustle of movement swept across the room.