Rashi, based on Targum Yonasan: Where none of you crouched or lied, i.e. outside of your land, you will be incarcerated. "Tachas" is a place, like "Shevu Ish Tachtav" (Shemos 16:29).

Radak: The Yud at the end of "Bilti" is a pronoun ¹ (Me), like "u'Moshi'a Ein Bilti" (Hoshe'a 13:4). Because the nation is without Me, they are not My nation; they do not heed Me. Therefore, it crouched under the incarcerated. I.e. in the place of the incarcerated, it crouched to be with them.

Radak citing Meforshim: The only one who will escape is one who crouches and hides among the incarcerated or the killed; [the enemy] will not think that he is alive.

Radak citing his father: Because they did not bow to Hashem, they will be incarcerated or killed.

Radak citing his brother R. Moshe: If not for the crouching in their land, that the Goyim make them submissive, a greater [affliction] would come on them, i.e. exile, in which some are incarcerated and some are killed.

Malbim: On a day when masses are killed, some seek to save themselves, and hand themselves over to the enemy in order to stay alive. If you want to save your lives, "to whom will you flee?" (3) Your only help is to crouch under the incarcerated (hand yourselves over to the enemy). Also refer to 10:4:2:3.

Rashi: In that place will fall those who were killed.

Radak: In the place of killed people, they will fall killed. I.e. some will be taken captive, and some will be killed; not one will remain in his land. All will be killed or exiled.

Malbim: On a day when masses are killed, some prefer to save their honor; they prefer death over falling into the enemy's hand. If so, "where will you abandon your honor?" (3) The only solution is "v'Sachas Harugim Yipolu" - to die on the battlefield, like a Gibor who risked his life; he is remembered honorably among the living.