During the course of World War I, he was deployed to the Western, Eastern, and Balkan fronts as an artillery observer with field aviation detachments.
In 1930 Kreiser and Rieseler went to the USA to join the Pennsylvania Aircraft Syndicate Ltd., headed by aviation pioneer E. Burke Wilford.
From 1925 to 1927 he also published numerous articles on aviation policy in the Berlin weekly Die Weltbühne and in Schairer's Sonntagszeitung under the pseudonym Konrad Widerhold.
On 12 March 1929 Kreiser published the article Windiges aus der deutschen Luftfahrt (‘Stormy Matters from German Aviation’) in the Weltbühne under the pseudonym Heinz Jäger.
According to his research, Lufthansa operated a flight division on the coast, and at the Johannisthal-Adlershof airfield there was a secret department M run by the German Aviation Company.
The article made the magazine known worldwide and resulted in charges being filed against Kreiser and the Weltbühne’s editor, Carl von Ossietzky.
The proceedings, known as the Weltbühne Trial, ended in 1931 with the two men being sentenced to 18 months in prison for betraying military secrets.
There Kreiser subsequently published details of the trial proceedings in the nationalist newspaper L'Echo de Paris.
Ossietzky strongly disapproved because the Reich Court, invoking the Espionage Act, had decreed that no details from the trial were to be made public.
Beginning in April 1932, Kreiser also published a series of exposés on the Reichswehr in L'Echo de Paris, presumably in conjunction with the pacifist Friedrich Wilhelm Foerster.