Royalty, political leaders and families of the fallen will unite in Belgium and the UK today in marking 100 years since Britain entered the First World War.
At 11pm on August 4, 1914, Britain declared war on Germany, ushering in four years of darkness, despair and appalling tragedy.
One hundred years later, on a national day of commemoration, events marking the anniversary of the start of the Great War will be held in London, Glasgow and Belgium.
In Glasgow, the Prince of Wales will attend a 10am service at the city's cathedral, followed by a wreath-laying service and march-past at the Cenotaph in George Square.
Prime Minister David Cameron, Sir Trevor McDonald and war reporter Kate Adie will deliver readings at the service which is being led by the Reverend Dr Laurence Whitley.
In London at 10pm - an hour before war was officially declared 100 years ago - a service of solemn commemoration will be held at Westminster Abbey.
Key figures there will include the Duchess of Cornwall, Deputy Prime Minister Nick Clegg, Labour leader Ed Miliband and Metropolitan Police commander Sir Bernard Hogan-Howe.
The Duke and Duchess of Cambridge and Prince Harry will be among 500 guests who will attend a twilight ceremony at St Symphorien military cemetery near Mons in Belgium.
Until the armistice was signed on November 11, 1918, millions of lives were lost, including 750,000 British and Commonwealth soldiers, in what was the bloodiest conflict the world had known.
The world's last known First World War service member, Florence Green, died aged 110 in King's Lynn, Norfolk, in February 2012.