LONDON — With a dimming of the lights and ceremonies across this country and in Belgium, monarchs, princes, presidents and citizens commemorated on Monday the day 100 years ago when Britain entered World War I at the start of four years of carnage once called the war to end all wars.

Some took the moment to recall more modern crises in the Middle East and Europe that are rooted in the fighting between 1914 and 1918 that toppled empires and redrew the world map. Some dwelled on a vision of reconciliation among former foes.

“We were enemies more than once in the last century, and today we are friends and allies,” Prince William, the second in line to the British throne, told a ceremony in Belgium, referring to Germany and its allies in two world wars. At Westminster Abbey, prayers were said in English and German.