Where is Tifsach?
Radak: It is not from Eretz Yisrael. Rather, it is across the [Yarden] river, in Aram. Shlomo ruled "mi'Tifsach v'Ad Azah" (Melachim I, 5:4). It seems that it was on the border of Yisrael, opposite Tirtzah. Tirtzah was Menachem's city, in Eretz Yisrael.
What is the meaning of "Ki Lo Pasach"?
Rashi: The head of the city did not open the city to him, to accept the king on themselves. Radak - he offered Shalom to them if they will accept to pay taxes to him; they refused.
Malbim: When one makes war against a city, one may not surround it from four sides. One must leave one side open from which one who wants to flee, may flee 1 (Rambam Hilchos Melachim 6:7). (Menachem transgressed.) Also, we do not kill women and children (ibid., 6:4).
So it says in Midrash ha'Gadol Bamidbar 31:7.
What are "v'Es Gevuleha mi'Tirtzah"?
Radak: It is everything from the outskirts of Tirtzah until Tifsach.
Malbim: It is the outskirts of Tifsach on the side of Tirtzah.
What are "he'Haroseha"?
Rashi: The pregnant women 1 inside (he cleaved them) 2 . Radak - he did so for vengeance and disgrace. Malbim - also this was unlike war etiquette and Torah law.
Ralbag: They are the strong towers in the mountains around it.
Radak: Even though the suffix specifies (its pregnant women), there is also the prefix Hei to identify. This is like "ha'Ahali" (Yehoshua 7:21).
Elisha cried because he saw through Ru'ach ha'Kodesh that Chaza'el Melech Aram would do so to Yisrael (8:12). We do not find that he did so. Perhaps because Yehu eradicated Ba'al and its worshippers from Yisrael (10:21-28), the decree was reversed onto Aram! (PF)