John Gill Commentary Deuteronomy 20:12

"And if it will make no peace with thee, but will make war against thee, then thou shalt besiege it:" — Deuteronomy 20:12 (ASV)

And if it will make no peace with you
Will not accept of terms of peace offered:

but will make war against you ;
come out and fight, or prepare to defend themselves: then you shall besiege it; surround and block it up on all sides with their forces; the Jews say only on three sides, leaving one for any to flee and make their escape if they thought fit; (See Gill on Numbers 31:7).