He may also have been misled by the fact that Principal Souza did not neglect to make the utmost capital out of the affair, thereby increasing the difficulties with which Lord Wellington was already contending as a result of incompetence and deliberate malice on the part both of the ministry at home and of the administration in Lisbon.
Indeed, but for these factors it is unlikely that the affair could ever have taken place at all.

If there had been more energy on the part of Mr.Perceval and the members of the Cabinet, if there had been less bad faith and self-seeking on the part of the Opposition, Lord Wellington's campaign would not have been starved as it was; and if there had been less bad faith and self-seeking of an even more stupid and flagrant kind on the part of the Portuguese Council of Regency, the British Expeditionary Force would not have been left without the stipulated supplies and otherwise hindered at every step.
Lord Wellington might have experienced the mental agony of Sir John Moore under similar circumstances fifteen months earlier.

That he did suffer, and was to suffer yet more, his correspondence shows.

But his iron will prevented that suffering from disturbing the equanimity of his mind.

The Council of Regency, in its concern to court popularity with the aristocracy of Portugal, might balk his measures by its deliberate supineness; echoes might reach him of the voices at St.Stephen's that loudly dubbed his dispositions rash, presumptuous and silly; catch-halfpenny journalists at home and men of the stamp of Lord Grey might exploit their abysmal military ignorance in reckless criticism and censure of his operations; he knew what a passionate storm of anger and denunciation had arisen from the Opposition when he had been raised to the peerage some months earlier, after the glorious victory of Talavera, and how, that victory notwithstanding, it had been proclaimed that his conduct of the campaign was so incompetent as to deserve, not reward, but punishment; and he was aware of the growing unpopularity of the war in England, knew that the Government--ignorant of what he was so laboriously preparing--was chafing at his inactivity of the past few months, so that a member of the Cabinet wrote to him exasperatedly, incredibly and fatuously--"for God's sake do something--anything so that blood be spilt." A heart less stout might have been broken, a genius less mighty stifled in this evil tangle of stupidity, incompetence and malignity that sprang up and flourished about him on every hand.