Yet further to increase this force, Cesare issued an edict during his brief sojourn at Spoleto ordering every house in the Romagna to supply him with one man-at-arms.
It was whilst here--as he afterwards wrote to the Pope--that news reached him that Guidobaldo da Montefeltre, Duke of Urbino, was arming men and raising funds for the assistance of Camerino.

He wrote that he could not at first believe it, but that shortly afterwards--at Foligni--he took a chancellor of Camerino who admitted that the hopes of this State were all founded upon Urbino's assistance; and later, a messenger from Urbino falling into his hands, he discovered that there was a plot afoot to seize the Borgia artillery as it passed through Ugubio, it being known that, as Cesare had no suspicions, the guns would be guarded only by a small force.

Of this treachery the duke strongly expressed his indignation in his letter to the Pope.
Whether the matter was true--or whether Cesare believed it to be true--it is impossible to ascertain with absolute conviction.

But it is in the highest degree unlikely that Cesare would have written such a letter to his father solely by way of setting up a pretext.

Had that been his only aim, letters expressing his simulated indignation would have been in better case to serve his ends had they been addressed to others.
If Guidobaldo did engage in such an act, amounting to a betrayal, he was certainly paid by Cesare in kind and with interest.