How would NATO respond if Russia attempted to invade NATO territory?

According to NATO Secretary General Mark Rutte, if Russia attempted to capture even one square mile of NATO territory such as Estonia or Poland, the reaction would be devastating. NATO's response would be immediate and severe, sending a clear message that any aggression against member states will not be tolerated. However, Rutte also expresses concern about NATO's long-term defensive capabilities. He worries that within three to five years, given Russia's massive defense spending and industrial base, NATO might struggle to defend itself adequately. This highlights the urgency for increased defense investments among European NATO members.