During the war between the Pandavas and Kauravas, Lord Krishna promised not to fight from any side or carry any arms. Bhishma, Lord Krishna's Devotee, challenged Him that he will make Him fight in the war.
Lord Krishna broke His promise for His devotees. Duryodhana once incited Bhishma that he wasn't fighting to his best because he loves the Pandavas, his grandchildren. Bhishma got terribly disturbed on this charge and declared that next day he would kill all the Pandavas.
Next day, Bhishma fought ferociously and within the fight, Arjuna's chariot got broken. At that point, Arjuna became fully unarmed and helpless. Then, Lord Krishna took up the chariot wheel and went straight to Bhishma so that he can save Arjuna. At seeing, Lord Krishna coming towards him, Bhishma wholeheartedly threw his weapons away. Lord Krishna by this act of running with the chariot wheel, broke His promise, to not carry any form of arms and thus He saved 2 of his great devotees (Arjuna & Bhishma) from perishing and winning Him with their Devotion.
So GOD might break His promise however he would never break his devotee's promise. Thus anyone who has resorted to Lord Krishna can never be destroyed & Lord can defend him perpetually.