It was 80 years ago when Adolf Hitler led German forces to Poland, which triggered the Second World War.
On 1 September 1939, the German Luftwaffe, the name of the German air force, began to bomb the city of Wieluń in Poland.
It was an attack that killed thousands of people and was designed to inflict terror amongst the civilian population.
After the bombings, Britain gave Germany an ultimatum to cease military operations.
But that was ignored, which triggered Britain and France to declare war on Germany on 3 September.
It was the start of a six-year conflict that killed tens of millions of people and soldiers.
Speaking at a ceremony on Sunday in the city of Wieluń, German President Frank-Walter Steinmeier asked Poland for forgiveness.
Mr Steinmeier said:
"I bow my head before the Polish victims of Germany's tyranny. And I ask forgiveness."
Sirens sounds were played at the ceremony to mark when the first bomb fell on the Polish city.
A minutes silence was also held to remember the victims.
During WWII, Poland suffered some of the worst casualties of the conflict with around six million civilian deaths.
Commemorative events are also taking place in Warsaw and the former military post of Westerplatte.
Shortly before 5am on 1 September 1939, the German battleship Schleswig-Holstein opened fire on a garrison of Polish soldiers stationed on the Westerplatte peninsula.
World leaders have been gathering at events in Poland, Prime Minister Boris Johnson said in a message:
"The Poles fought magnificently for the Allied cause."
Cover image: Nazi German battleships attacked a Polish military base on September 1, 1939 (Picture: PA).