Who pursued whom?
Rashi: 'How one of the enemy could pursue a thousand of Yisrael, and two, ten thousand.' 1
Hadar Zekenim (Vayikra 26:8, based on Mo'ed Katan 16b): Also here discusses Yisrael chasing them, i.e. Adinu ha'Etzni (David). He killed 800 at once (Shmuel 2:23:8), and not 1000, due to the sin with Uriyah. 2
Riva (Vayikra 26:8, citing R"M of Kutzi): Here it says that the enemy should ponder that "b'Acharisam", in the days of Mashi'ach, one Yisrael will chase 1,000, and two will chase 10,000. .
See Sifsei Chachamim.
Kol Eliyahu: Rashi (16b) says that 200 were taken from him due to the sin. What is his source that initially he killed 1,000? Maseches Sofrim 8:1 lists the differences between his Shirah in Shmuel (2:221-51) and in Tehilim (18:1-51). In Shmuel (which was first) it says in verse 44 "Tishmereni"; its Gematriya is 1000. In the latter, it says "Tesimeni"; its Gematriya is 800.
What is the translation of the latter half of the Pasuk "Im Lo ki Tzuram Mecharam ... "?
Rashi and Targum Yonasan: 'If their Rock (Hashem) had not sold them out 1 and Hashem had not handed them over to us (finished them off - Targum Yonasan)'.
Hadar Zekenim: "Tzuram" is Avraham 2 , At Bris Bein ha'Besarim, Hashem showed him a smoky oven and a fire, to hint to subjugation under the kingdoms, and Gehinom. His seed is destined to suffer one of them; Hashem asked him to choose one. He chose for (sold) them to be subjugated.
QUESTIONS ON RASHI
Rashi writes that one enemy will chase 1,000 Yisraelim, and two will chase 10,000. Regarding Brachah, five Yisraelim will pursue 100, and 100 will pursue 10,000 (Vayikra 26:8). If so, the Midah of punishment exceeds Midah Tovah, unlike Sotah 11a says!