The question that had to be faced in 1556 was as simple to formulate as it was difficult to answer: under what conditions could Philip take up arms against the pope? The problem was to ensure that the king took part in a “just war”, but necessity dictated a pre-emptive strike against the papal states. The theologians he consulted replied almost to a man in the strongest, clearest, most negative terms. By definition, no war against the papacy was just. Even if the pope was at fault, as they were willing to admit, Philip must not fight against him. If he did, he would be accused of being a bad Christian, if not indeed a heretic, which would cause him to lose reputation. Furthermore, if Philip took up arms against the pope it might deal a mortal blow against the papacy which had been so seriously weakened by the onslaughts of heresy and worse might follow. In the words of Domingo de Soto, if Philip attacked Paul, he could inadvertently end up ‘eliminating the faith’. (..)
Ranged against Alba and the theologians were those who believed that defence was always a just cause, and pre-emptive strikes justified under pressing circumstances such as this. Philip would fail in his duty to his subjects if he remained inactive and allowed his enemies to gather their forces and attack his lands. Better keep the fighting away from his troubled and divided states, particularly in areas like Naples where pro-French sentiments endured. It would be cheaper and more effective to attack before the Franco-papal forces had time to coalesce. Such tactical and financial considerations won the day, but they were not the only reasons why Philip eventually attacked. As some of his counsellors pointed out, there was nothing in the military or financial situation of his empire to attract new allies, and little to persuade existing allies to remain within his camp. Philip’s sole international success to date had been to cement his friendship with Genoa by offering help to recover Corsica. The reputation he had established as a king willing to back his allies in their time of need was being undermined and would be lost if he failed to help Colonna and the other victims of Paul’s aggression. (..)
Philip’s attitude towards the war had been ambivalent throughout. He found it abhorrent to take up arms against the papacy, having considerable respect for the institution if not for its incumbents. He was a very traditional man, and it disturbed him to see the world upside down, with the Catholic monarch on one side; the pope, Most Christian king and Ottoman sultan on the other. He was deeply upset by Paul’s accusations of rebellion, perhaps because he was terrified that it would deal the death blow to his much weakened authority in some of his troubled states. But he felt very strongly that the action was unjust and dishonourable. From a letter written to Juana during June-July 1557 it is also evident that he was astonished as well as upset to find himself in this predicament. He thought it inconceivable that the pope should attack him – the man who had so distinguished himself in the restoration of England to the fold.
Maria José Rodriguez-Salgado, The Changing Face of Empire: Charles V, Philip II and Habsburg Authority 1551-1559