Percival had read all about the incident with Grindelwald in Paris, both in the official reports and in the wizard newspapers. He had always found reading the two together gave a fuller picture of what had truly happened, with the dourness of the official report counterbalanced against the sensationalism of the wizard piece, where all sorts of 'witnesses' came forward to speak with the Press who would not have dared speak to an Auror. It all came down to one thing. A new war had begun and all of the wizarding world would eventually have to choose sides. If it were just a simple matter of the magical communities living openly alongside the non-magical world of humans then Grindelwald was powerful enough he could have accomplished that alone, with no need of an army.
What Grindelwald craved was power.
He did not see the No-maj as equals and wanted to rule over them, and having seen the devastation of the Great War first hand Percival could sympathize. He had been swept up into that same war when parts of the wizard world tried to take advantage of the unfolding events to their advantage. He had met Theseus Scamander on the front line, fighting side-by-side, and was saddened to read of the loss of his fiancee, Leta Lestrange. He knew of the younger brother, Newton, but from all the accounts of the younger Scamander he had never expected to learn it was Newton who had revealed Grindelwald's subterfuge in New York.
After he was captured Grindelwald refused to reveal whether Percival was dead or alive, and no witch or wizard was strong enough to force him. He was certainly a powerful enough wizard to not require Polyjuice to hold onto Percival's form, and they all knew he would only keep Percival alive so he could tease out any memories - if he even needed them. After discovering the identity theft Seraphina had ordered no stone unturned as they searched for him, and fortunately they found him in the basement of his own home, locked within an extension charm through a crack in the brickwork. Apparently he had the younger Scamander to thank for that too, for the loan of a magical beast that could detect such a charm, otherwise he might have had to endure his imprisonment for far longer - or died in that cell.
Percival's first action once he was allowed to return to duty was to set up a system to ensure no other imposters could walk into MACUSA's headquarters undetected.
His second was to expose Grindelwald's supporters hidden inside MACUSA, especially those who had assisted in Grindelwald's escape.
His third was a personal request to join the hunt for Grindelwald in Europe, and under the circumstances Seraphina owed him this favor.
This was how he ended up fighting alongside Dumbledore and the two Scamander brothers, and how he and Newt Scamander discovered an attraction between them that even Grindelwald's impersonation of him could not deter or mar. The war had just begun and no matter that he was pure blood, or that he shared some of Grindelwald's views on the No-maj, as Percival looked into Newt's eyes he knew he would always choose Newt's side.