Remembrance Awakens Redemption
בפסוק כי תאמר כו' רבים הגוים האלה כו' רש"י פי' בלשון שמא תאמר
On the verse "When you will say in your heart, these nations are more numerous than I" (Devarim 7:17), Rashi explains the word "ki" in the sense of "perhaps you will say."
Rashi reads the opening "ki" of the verse as introducing a hesitation — "perhaps you will say" — that the verse goes on to address.
ומו"ז ז"ל פי' בלשון אף שהוא הבטחה אף שלא יתחזקו כ"כ באמונה מ"מ יתחזקו בהקב"ה
And my grandfather and teacher, of blessed memory, explained the word "ki" in the sense of "even though" — meaning it is a promise: even though Bnei Yisrael will not strengthen themselves so much in emunah, nonetheless they will be strengthened through the Holy One, Blessed is He.
The Sfas Emes's grandfather read the same "ki" as "even though," turning the verse into a promise: even when their emunah is weak, Bnei Yisrael are carried by the Holy One, Blessed is He.
ולכן נאמר מעט מעט אגרשנו כו' וי"ל עוד כפשוטו כי כשיפול בלב בנ"י פחד מהאומות לא יאמרו כי הוא סימן ח"ו שיוסיפו עי"ז לירא מהם
And therefore it is said "little by little I will drive them out" (Devarim 7:22); and one may further explain it according to its plain meaning, that when fear of the nations falls into the heart of Bnei Yisrael, they should not say that it is a sign, Heaven forbid, that they will thereby come to fear them all the more.
On the plain level, the gradual conquest teaches that when dread of the nations enters a Jew's heart, he should not assume it is a bad sign that will only deepen his fear.
כי באמת צריכין בנ"י להשגיח על כל מה שנופל בלבם
For in truth Bnei Yisrael must pay close attention to everything that falls into their hearts.
This is because Bnei Yisrael are obligated to take seriously every feeling and thought that arises within them, never dismissing it as meaningless.
אך הכתוב אומר כי הוא רק כדי לזכור ביצ"מ עי"ז נופל בלבם פחד המלחמות כדי שיזכרו חסדי ה' שעשה עמנו במצרים וזכירה זו עצמה היא העצה להינצל מהם וכתי' זכור תזכור כו' שע"י הזכירה מעוררין זכירה בשמים ג"כ שבנ"י מעוררין גאולת מצרים ע"י הזכירה כנ"ל:
Rather the verse is saying that it is only in order to remember the exodus from Mitzrayim — through this there falls into their hearts a fear of the wars, so that they should remember the kindnesses of Hashem that He performed with us in Mitzrayim; and this very remembrance is itself the counsel by which to be saved from them, as it is written "you shall surely remember" (Devarim 7:18), for through the remembrance one arouses remembrance in the heavens as well, that Bnei Yisrael arouse the redemption from Mitzrayim by means of the remembrance, as above.
The dread of the nations is actually planted in the heart to trigger remembrance of the exodus and Hashem's kindnesses, and that very act of remembering is the salvation — for our remembering below awakens Hashem's remembrance above, drawing down the power of the geulah from Mitzrayim.
Summary: The Sfas Emes examines the verse "When you will say in your heart, these nations are more numerous than I," weighing Rashi's reading of "ki" as "perhaps you will say" against his grandfather's reading of it as "even though" — a promise that Bnei Yisrael are sustained by the Holy One, Blessed is He, even when their emunah falters. He then offers a plain reading: when fear of the nations enters the heart of Bnei Yisrael, they should not treat it as an ominous sign, for in truth they are obligated to attend carefully to everything that falls into their hearts. Rather, that very dread is implanted so that they will remember the exodus from Mitzrayim and the kindnesses Hashem did for us there. And this remembrance is itself the salvation, for "you shall surely remember" — our remembering below awakens a corresponding remembrance in the heavens, drawing down the power of the geulah of Mitzrayim to save Bnei Yisrael from their enemies.