The crime of assault generally requires that both the perpetrator and the victim of an assault be a natural person. Thus, unless the attack is directed by a person, an animal attack does not constitute an assault. However, under limited circumstances the Unborn Victims of Violence Act of 2004 treats a fetus as a separate person for the purposes of assault and other violent crimes.Possible examples of defenses, mitigating circumstances, or failures of proof that may be raised in response to an assault charge include:

Lack of intent: A defendant could argue that since they were drunk, they could not form the specific intent to commit assault. This defense would most likely fail, however, since only involuntary intoxication is accepted as a defense in most American jurisdictions. Mutual consent: A defendant could also argue that they were engaged in mutually consensual behavior. For example, boxers who are fighting in an organized boxing match and do not significantly deviate from the rules of the sport cannot be charged with assault. State laws
Laws on assault vary by state. Since each state has its own criminal laws, there is no universal assault law. Acts classified as assault in one state may be classified as battery, menacing, intimidation, reckless endangerment, etc. in another state. Assault is often subdivided into two categories, simple assault and aggravated assault. Simple assault involves an intentional act that causes another person to be in reasonable fear of an imminent battery.
The Offences against the Person Act 1861 creates the offences of:

Common assault and battery: a summary offence, under section 42;
Aggravated assault and battery: a summary offence, under section 43
Common assault: under section 47
Assault occasioning actual bodily harm: under section 47The Criminal Justice (Miscellaneous Provisions) Act (Northern Ireland) 1968 creates the offences of:

Assault with intent to resist arrest: under section 7(1)(b); this offence was formerly created by s.38 of the OAPA 1861.That Act formerly created the offence of Assault on a constable in the execution of his duty. under section 7(1)(a), but that section has been superseded by section 66(1) of the Police (Northern Ireland) Act 1998 (c.32) which now provides that it is an offence for a person to, amongst other things, assault a constable in the execution of his duty, or a person assisting a constable in the execution of his duty. United States
In the United States, assault may be defined as an attempt to commit a battery. However, the crime of assault can encompass acts in which no battery is intended, but the defendants act nonetheless creates reasonable fear in others that a battery will occur.Four elements were required at common law:
The apparent, present ability to carry out;
An unlawful attempt;
To commit a violent injury;
Upon another.As the criminal law evolved, element one was weakened in most jurisdictions so that a reasonable fear of bodily injury would suffice. These four elements were eventually codified in most states.