Fellers was the US Military attache in Cairo and he was given every help by the British, but Fellers transmitted his reports to Washington using an old cipher code-called 'Black' code for communications to and from overseas embassies, which had been completely compromised. The Italians had broken into the US Embassy in Rome, in 1941 copied the code book and returned it without the Americans knowing anything.
OK, so what effects does it have on German successes in North Africa if Fellers' objection to using the State department code had been accepted?