The conversation took place during the battle of Banu Mustaliq according to traditions. And the hypocrites are not warning about bringing any army, they are inciting the Ansar to drive away Muhajirs from their homes when they return to Medinah.
Islahi sahab writes, “Tradition has it that on the occasion of the battle of Bani Mustaliq in 6 AH, a quarrel broke out between a poor Muhajir and an Ansari over water at the spring where the Prophet (sws) encamped. Muhajir slapped Ansari. Ansari called the Ansar and Muhajir gathered the Muhajir. The men on both sides came together, brandishing swords.
Although, by the grace of God Almighty, the matter did not escalate, Abdullah ibn Abi, who was always on the lookout for such opportunities, took advantage of the situation and uttered very poisonous phrases to provoke the Ansar against the Muhajireen. “They have taken refuge in our house and are now showing teeth to us,” he said. “Truth it is, whoever has said, make the dog fat, and it will eventually bite you. By God, if we return to Medinah now, those who are honorable among us will drive the wicked (muhajirs) out of there”.