لَّقَد تَّابَ اللَّهُ عَلَى النَّبِيِّ وَالْمُهَاجِرِينَ وَالْأَنصَارِ
“Allah has already forgiven the Prophet and the Muhajirin and the Ansar.”
Names the "time of difficulty" (sa'at al-usrah) from which Jaysh al-Usrah, the expedition's other name, is drawn.
Revealed about Allah turning in mercy to the Prophet ﷺ, the Muhajirun, and the Ansar who stood by him at Tabuk.
This is the specific, extremely well-documented asbab al-nuzul: Ka'b ibn Malik's own account (quoted in the post) of Allah's mercy after 50 days of being shunned -- the direct occasion for this ayah's revelation.