French police are preparing to put on display their secret files on Pablo Picasso for the first time.
Picasso was deemed "unfit for naturalisation"
These include Picasso's little-known application for French citizenship - rejected because of his "extremist ideas and drift towards communism".
Documents show that Picasso was spied on initially as a suspected anarchist, and later over his communist sympathies - before he became a prominent member of France's Communist Party.
The exhibition opens at the Paris police museum on Tuesday.
The files travelled around Europe before making their way back to Paris.
During World War II the occupying Nazis took them to Germany, along with thousands of other police files.
Russians transferred them to Moscow when they captured Berlin in 1945. In 2001, they were returned to Paris.
"I received 140 cardboard boxes," Claude Charlot, the police department's archives director, said.
He told France's Liberation newspaper how one day a Russian official charged with compiling the documents came across Picasso's citizenship application, dated 3 April, 1940.
"It was a big surprise."
"We read, we re-read. It was not a fake."
He gave the documents to two Picasso experts, Pierre Daix and Armand Israel, who published them in a book last October.
Spanish-born Picasso probably applied for French citizenship because he feared extradition to General Franco's fascist regime if the Nazis invaded France.
But he never received a reply, because he was considered "suspect from a national viewpoint".
"Despite having set himself up in France as a so-called modern painter, allowing him to earn millions of francs and to own a chateau near Gisors, Picasso has maintained his extremist ideas and drifted towards communism," a report quoted by Liberation says.
"(The artist) has no right to be naturalised."
Other documents in the archives show how French police had trouble gathering information about Picasso when he first arrived in Paris in 1901, because he barely spoke French.
Agents reported that the young painter never attended any anarchist meetings and that the caretaker in his building "never heard him express any subversive opinions".
But "it is reasonable to assume he shares his flatmate (Pedro) Manach's anarchist opinions," a police chief wrote.
Anarchists in France were under close scrutiny at the time, following a bombing campaign a few years earlier.
Picasso was born in Malaga, in southern Spain, in 1881.
He vowed not to return to Spain while General Franco was in power.