וַיִּזְכֹּר לָהֶם בְּרִיתוֹ וַיִּנָּחֵם כְּרֹב חֲסָדָו
He remembered His covenant for them and He relented according to His great mercy.
The Metzudas Dovid explains that this refers to the covenant that G-d forged with our Patriarchs as per Leviticus chapter 26: G-d will recall the covenant He made with Avraham, Yitzchak and Yaakov (verse 42) and He will not destroy them because of it (verse 44). He does this not because of any merits the nation may possess but because of His own great mercy. (This is one of the verses of remembrance recited as part of the Rosh Hashana Musaf service.)