During his visit to the Philippines in December 1962, Guerrero discussed with de la Costa a project on nineteenth century Philippines and along with it was the objective of recovering the Burgos trial records. In London before he left for Madrid, he saw the Spanish ambassador, the Marquess de Santa Cruz, who suggested to him to get in touch with key Spanish officials. In the first quarter of 1963, he inquired about the Burgos trial records and communicated to the Army Ministry but they were evasive.

Once the records were available to the public, the rewriting of Philippine history, at least from 1872 onwards, would be logical. The trial of Burgos was still shrouded in mystery for almost a century because no Filipino or even Spanish historians had accessed the trial records. Gaining access to these procesos would be boon to their project. Towards this end, he reread Montero y Vidal's Historia General de Filipinas, which took him to the movement of Apolinario de la Cruz and recognized its Katipunan resemblance, particularly its restriction to Tagalogs, making him conclude that Filipino nationalism was based on racial resentments. Rereading the same, he realized almost on the eve of the Philippine Revolution, that the Spanish conquest never succeeded in the mountainous north and in the southern islands. On Montero y Vidal's account, he said it “reads just as badly as Zaide et al. sometimes; all those decrees — but his footnotes are often amusing and some of his accounts of historical events most readable.” After Montero y Vidal, he wanted to read Sinibaldo de Mas’ Informe sobre el estado de las Islas Filipinas en 1842, a copy of which he was looking for, but he had instead Martinez de Zuñiga's Estadismo de las Filipinas. For the Spanish background of nineteenth century Philippines, he enjoyed La Política y Los Políticos en el Reinado de Carlos III, leading him to two other books.

In all these readings, he began to question the beginning of the nineteenth century in the Philippines: