Radak: Pelishtim ruled over Yisrael for many years. Even in the days of Shimshon, who saved Yisrael, they ruled for many years - "ha'Lo Yadata Ki Moshlim Banu Pelishtim" (Shoftim 15:11)! Even though in the days of Shmuel, the Pelishtim were subdued and returned the cities that they had taken from Yisrael, after Shmuel grew old, Yisrael were under their rule. The Pelishtim had Netzivim in Eretz Yisrael, and removed all iron smiths to Eretz Pelishtim. There was not even a smith to make Kelim for plowing or other professions! Perhaps some Yisraelim had swords or spears from beforehand, but none of Sha'ul's 600 men or Yonason's 1,000 had one. However, they had bows, clubs and slingshots; most mighty men at that time fought with arrows and rocks, like it says about David's men "Mayminim u'Masmilim ba'Avanim uva'Chitzim ba'Keshet" (Divrei ha'Yamim 1 12:2), about Milchamas Mo'av "va'Yasobu ha'Kala'im va'Yakuha"(Melachim 2 3:25), and about David "v'Kal'o v'Yado" (17:40). Even so, it was a great shortcoming that the only sword or spear was with Sha'ul and Yonason. Salvation is only through Hashem! Also, Yisrael had only 1,600 men, and Pelishtim had 36,000 chariots, 6,000 horsemen and [foot] soldiers like the sand of the sea.

Radak: It started in the days of Shimshon.

Malbim: It was from when Sha'ul became king.

Radak: The way it is written (singular) refers to the Klal, and the way we pronounce it (plural) refers to the Prat.