Ahead of Germany's federal elections, the BBC talks to people from different backgrounds about their lives in modern Germany. In the fourth part of our series, an old-age pensioner living in the former East Germany recalls his austere existence, which spanned four eras of Germany history.
For a man of 92, Mr Sonntag is light on his foot - his right foot, the one which came back from the Russian Front.
He hops off a low wall on our way around the city gardens where we meet, and scurries up all four floors to his flat, clutching his shopping bags.
Bright as a uniform button, he waits for the interview to begin, without taking so much as a drop of water despite the late summer heat. Once they had so little to drink in the Caucasus, he tells me, he and his fellow German soldiers even sucked the dew off pebbles in the mornings.
Anything may be borne but thirst, the old infantryman says. His story - what he is prepared to tell of it, at least - is one of loss and forbearance.
Mr Sonntag was born in 1921 in Chemnitz, an important textile city near the border of what is now the Czech Republic. Its population reached a peak of 360,000 in 1930.
During the tumultuous years of the Weimar Republic (1919-33), the young Arno had a very modest upbringing, as hyper-inflation devoured German savings.
Under the Third Reich (1933-45), he was called up to serve in the army like millions of other Germans during World War II.
He had no choice but to serve, he says, blaming Nazi atrocities on Heinrich Himmler and the SS.
In another hot summer, 70 years ago this year, near the Russian city of Oryol, Corporal Sonntag was injured by a Soviet grenade, losing his left leg and receiving a splinter in his eye.
It took him out of the battle that would be remembered as Kursk, the fight that broke Nazi Germany's hold in the East.
By the time of the final defeat in 1945, Chemnitz had bitten the dust of total war, pulverised by British and US air raids.
'Do this, do that'
Sweeping up the ashes, the new, Soviet-dominated German Democratic Republic (1949-1990) built what was intended to be a model city of socialism, with a communist name to match: Karl-Marx-Stadt (Karl Marx City).
For Mr Sonntag too, it was time to rebuild his life in his old town. With the help of a veterans' hospital, the war invalid found a job in a foundry.
"In the GDR they always told you what to do," he says. "You had to go here, you had to do this. You couldn't buy this or that."
He laughs as he shows me a miniature replica he once made of the giant bust of Karl Marx which dominates the city centre to this day.
"It's different now," he says in his flat in the suburbs of what, since reunification under the Federal Republic of Germany, is once again Chemnitz.
"Now we have freedom. Everybody can do what they want. You can travel wherever you want - if you have the money!"
The GDR-era streets of Chemnitz have taken on a Western air with new shopping malls and cafes, but the city feels curiously empty.
In fact, since reunification and with migration westwards, its population has dropped to just 245,000. Some years ago it was said to have the lowest birth rate in the world.
Himself childless, Mr Sonntag lost his beloved wife Elfrieda 10 years ago. He keeps their wartime wedding photo, which shows the decorated soldier in uniform, alongside his identity card.
Finding himself on a new path again, the widower learnt to cook and keep house. All is spotless, orderly and cheerful within.
Seven years ago came the modern world, when he acquired a laptop.
Living on a pension of 900 euros (£757; $1,197) a month, he leads a frugal lifestyle, shopping at a budget supermarket.
He clearly has good friends and likes to spend time in the city centre.
Like many people of his generation across Europe, Mr Sonntag extols the virtues of moderation, resents the consumer society, and worries about immigration and distant conflicts.
A cuckoo clock dating from the 1920s still keeps time in the flat. It is a prize the young Arno won when shooting was still only a boy's game, and one of the few objects he possesses that is nearly as old as himself.
"We are lucky that it has been quiet in Germany since 1945," he says.