The relationship between God and the Israelites operates on a strict principle of measure for measure, where the consequences the people face are a direct reflection of their own betrayal. A dual reality exists in how this punishment unfolds. On one hand, the people brought this harsh reality upon themselves through their own deliberate actions [רד"ק]. At the same time, God takes an active role in the response [מנחת שי], bringing about a severe judgment that perfectly matches the gravity of the Israelites' deeds [רש"י, מצודת דוד, ביאור שטיינזלץ].
The core offense that triggers this devastation is the contempt shown toward a sworn promise and the shattering of a sacred covenant [מצודת ציון, ביאור שטיינזלץ]. The primary approach among commentators is that this points back to the original agreement the Israelites accepted at Mount Sinai when they received the Torah. This relationship was strictly conditional, relying entirely on their continued observance of the laws and commandments.
Because the people treated their oath with contempt and broke the agreement through their sins, God responds by voiding the covenant from His side. This nullification results in the tragic loss of the gifts that were dependent on their faithfulness, namely the Land of Israel, the Temple, and their kingdom [מלבי"ם].
Yet, even in the midst of this severe judgment, a fundamental difference emerges between God and His people. Although God punishes the Israelites exactly as they deserve for violating their agreement, He will not destroy the covenant entirely, nor will He ever erase it from His memory. He remains faithful, choosing not to act in the same faithless manner as they did [רש"י].