During his father's reign only the county of Sens, vacant through the death without heirs of Count Renard (Reginhard), had been (in 1055) re-united to the crown, an important acquisition, but one for which King Robert himself had prepared the way, by separating in 1015 the county of Sens from the duchy of Burgundy: thus it cost Henry no effort whatever. Philip had no sooner taken the reins than an opportunity arose for him to link together his possessions in the Orléanais and the Sénonais by making himself master of the county of Gâtinais. Geoffrey the Bearded, who bore the title of its Count, and had succeeded his uncle, Geoffrey Martel, in the county of Anjou (1060), had just been imprisoned by his brother Fulk Rechin, who had usurped power in both counties. Philip, without hesitation, joined a coalition formed by the Count of Blois and the lords of Maine against the usurper, and, as the price of peace, exacted the cession of the county of Gâtinais (1068).
A few years later he used the minority of Simon of Crépy, Count of Valois and Vexin, as an opportunity to fall upon his estates. These were very extensive, comprising not only the Vexin and Valois, but the county of Bar-sur-Aube and the territory of Vitry-en-Perthois, which Simon's father, Raoul III of Valois, had acquired by marriage, and, on the north, the county of Montdidier, and Péronne which he had taken from the Count of Vermandois. Entrusting to his vassal, Hugh Bardoux, lord of Broyes, the task of seizing Simon's possessions in Champagne, Philip invaded his other domains in 1075. For two years the struggle went on, almost without a break, fiercely and pitilessly. At last, in the beginning of 1077, the unlucky Simon was forced to beg for peace, and to cede to the king the county of Vexin.
At about the same time, Philip claimed the town of Corbie, which had come to Baldwin of Lille, Count of Flanders, as the dowry of Adela, daughter of Henry I of England; and as Count Robert the Frisian refused to surrender it, he entered it by surprise and caused the inhabitants to swear fealty to him. Robert, confronted by an accomplished fact, after a brief attempt at resistance, found no resource but to submit. Corbie was never again to be detached from the royal domain.
Again, in 1101, Philip was to be seen profiting by need of money on the part of Odo-Harpin, Viscount of Bourges, who was about to set off for the Holy Land. The king enlarged the royal domain by purchasing from him an extensive district comprising, besides Bourges, the lordship of Dun-le-Roi.
Nearly all the enterprises of Philip I shew the same character, at once inglorious and practical. His chief efforts were in the direction of Normandy, where two parties confronted each other, on the one hand the King of England, William the Conqueror, and on the other, Robert Curthose, his son. Philip's entire policy consisted in supporting Robert, though he was ready, it would appear, to desert him as often as there seemed any prospect of his becoming dangerous: a course which did not
