He did not request Adam and Eve to put enmity into exercise. In fact, he was not even addressing Himself directly to them, for he was speaking to the serpent. Only God can do what we cannot do for ourselves. God takes the initiative. The fruit of God giving a new heart and planting a new enmity will always be conflict.
Spiritual life is a struggling, bruising battle.
It is holy warfare. The Lord never promised His Son or His people an easy way of salvation. Wherever God builds His church, Satan will build his temple next door. To the end of time, Satan will wage war against all that is of God and Christ. Satan is a fallen angel.