If you're going to be a hard-ass, you need to be consistent. I think Machiavelli makes that point twenty or thirty times in The Prince. Saint Pius V, baptized Antonio Ghislieri, may not have read The Prince, but he surely understood that principle well. [Pius V's predecessor, Pius IV, put added The Prince to the Index Librorum Prohibitorum, but that didn't stop folks -- even clergy -- from reading it.]
At fourteen, Antonio entered the Dominican Order. He was ordained at twenty-eight and spent the next sixteen years lecturing against the heresies of the day. He was pretty good at this, eventually scoring himself a position in the Inquisition. He also turned his attention back into the Dominican Order, smacking some of the more lax brothers back into line. His advancement continued through a couple of bishoprics, a cardinal's hat, and the supreme inquisitor's office. However, when he criticized Pope Pius IV (to his face, no less) for giving a cardinal's hat to a thirteen-year old relative and supporting a nephew out of the Vatican treasury, he lost a little ground. That sweet office in the Lateran Palace got reassigned to someone a little more... collaborative and his inquisitory (inquisitive?) powers were diminished.
The death of Pius IV was a tidal change in his fortune. Antonio generously took his predecessor's name when he was elected Pope in 1566, just ten days shy of his sixty-second birthday. His first order of business was to clean house in Rome. The prostitutes were expelled from the City. Expenses at the papal household were slashed. Residency requirements for clergy "working" at the Vatican were enforced. In short, the worst excesses of the Roman Catholic Church were curtailed or eliminated, causing some grumbling but not much active opposition within the lower Church ranks.
Of course, he took it to the Protestants as much as to his own flock. Huguenot clergy in France were excommunicated, as was Queen Elizabeth I of England. Probably none of those folks minded very much,as they were tossing around that phrase "Whore of Babylon" every time someone mentioned Rome.
If success is evidence of divine favor, then surely Heaven smiled down on Pope Pius V. If the Divine Watchmaker is letting the world spin without interference, then the credit for the victory goes to Pius and his allies. You see, the Ottoman Empire had been pushing into Europe for centuries, and pretty much having it all their own way. The Pope organized an alliance called the Holy League. Getting squabbling European princes to set aside the vain machinations is no easy task, but he did it. The Ottoman navy struck and was defeated at the Battle of Lepanto in 1571. Although Pius was in Rome, the Holy League's victory was revealed to him before any messengers reached the city.