The suspect, who was seized two days before France's presidential election run-off vote, is said to be psychologically unstable, according to a letter seen by AFP today.
He was arrested close to an airbase at Evreux, northwest of Paris by police who found guns hidden nearby and several IS flags in his car.
"My name is Alain Feuillerat, a Muslim soldier defending my fatherland," he said in the message seen by AFP.
"I was preparing, with Allah's help, an attack against the Evreux-Fauville airbase," in the one-and-a-half page letter which mixed criticism of France with religious statements.
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"This attack was not designed to kill the maximum number of people but above all to show our determination," he continued in the message which was dated April 27 but posted in the mail on May 5, the day of his arrest.
The computerised message had two photos attached. One showed the suspect dressed as an Islamic State group fighter, the other was a photo of his French passport.
Feuillarat, 34, remains in detention and was on Monday charged with a terrorist act and with attempting to break into a military area.
The former soldier quit the army in 2013 after ten years of service.
He converted to Islam and had been under close surveillance since 2014 due to his radicalisation.
France has been hit by a string of Islamist attacks, most claimed by IS, since 2015, with over 230 people killed in all.
The presidential election went off without a hitch on Sunday but the prior campaigning was overshadowed by security worries.
Karim Cheurfi, a 39-year-old Frenchman, shot and killed a police officer and wounded two others on the Champs Elysees avenue days before the first round of voting in April, in an attack claimed by the Islamic State group. Cheurfi was killed by police gunfire.
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