After the war, he was convicted by the Allies of "spoliation and plunder", but released almost immediately, and continued his career as a political lobbyist and business executive, becoming chairman of a Swiss chemical company.

[1] By 1934 Ilgner had been appointed as a deputy member of the Vorstand of IG Farben and had a remit that gave him some control over not only financial and public relations decisions but also the company's dealings with central government.

[1] Ilgner's rapid rise, his naked ambition and the feeling that he was being pushed only because of his family connections made him unpopular with many of his fellow executives but Schmitz held too much power in the company for such objections to be raised publicly.

[8] According to Ilgner's much later claims his influence amongst the Nazi hierarchy even saved the life of Gattineau on Night of the Long Knives after his fellow executive had been arrested due to his membership of the Sturmabteilung and his friendship with Ernst Röhm.

When the fate of the country was sealed and the Nazis took over IG Farben were rewarded for their part in events by being allowed to take control of Aussiger Verein, a Prague-based chemical company that was the fourth largest in Europe, at a low price.

[11] Immediately after the outbreak of the Second World War Ilgner handed over the company's extensive collection of geological maps to the government, informing them of the locations across Europe where chemicals might be extracted.

In fact Ilgner was simply trying to avoid falling into the hands of the Soviet Union and so moved westwards and whilst in Frankfurt ensured the disappearance of the incriminating documents he had taken with him.

Ilgner in particular was singled out for the leading role he played in German rearmament[15] and at the trial it was noted that he was the only executive who continued to fraternise openly with Carl Krauch who was bearing the bulk of the guilt due to his close relationship to Hermann Göring.