Antonyms of assault:
aid, cover, protect, shelter, support, uphold, befriend, defend, resist, shield, sustain, withstand, defense, defensive, guard, opposition, resistance, protection, security.
Examples of usage:
  1. He wanted to get away from his own dangerous situation of the moment in that office- and he wanted to protect the old man outside from assault.
    "When Egypt Went Broke", Holman Day.
  2. He was studying something by himself upon a figure which he had drawn, to which he had so utterly given up his thoughts and his sight that he did not notice the assault of the Romans and the capture of the city, and when a soldier suddenly appeared before him and ordered him to follow him into the presence of Marcellus, he refused to do so before he had finished his problem and its solution.
    "Plutarch's Lives, Volume II", Aubrey Stewart & George Long.
  3. You are under arrest, according to the Articles of Space, for conspiracy, armed assault....
    "Feline Red", Robert Sampson.