“And by golly, love sure was a battlefield. Benatar was right about that.”

1840s, Essays: First Series (1841), Compensation
Context: The league between virtue and nature engages all things to assume a hostile front to vice. The beautiful laws and substances of the world persecute and whip the traitor. He finds that things are arranged for truth and benefit, but there is no den in the wide world to hide a rogue. Commit a crime, and the earth is made of glass. Commit a crime, and it seems as if a coat of snow fell on the ground, such as reveals in the woods the track of every partridge and fox and squirrel and mole. You cannot recall the spoken word, you cannot wipe out the foot-track, you cannot draw up the ladder, so as to leave no inlet or clew. Some damning circumstance always transpires. The laws and substances of nature — water, snow, wind, gravitation — become penalties to the thief.
On the other hand, the law holds with equal sureness for all right action. Love, and you shall be loved. All love is mathematically just, as much as the two sides of an algebraic equation. The good man has absolute good, which like fire turns every thing to its own nature, so that you cannot do him any harm; but as the royal armies sent against Napoleon, when he approached, cast down their colors and from enemies became friends, so disasters of all kinds, as sickness, offence, poverty, prove benefactors:

"Love is a Battlefield" (co-written with Mike Chapman)

In 1959 during the filming of On the Beach.
This has become a standing joke about Melbourne, but it's almost certainly either apocryphal or a misquotation.
Misattributed

Metro, August 25, 2006 (When asked if he was trying to stop taking drugs)
Drugs

Source: The Wise Man's Fear

1940s, Religion and Science: Irreconcilable? (1948)
Context: While religion prescribes brotherly love in the relations among the individuals and groups, the actual spectacle more resembles a battlefield than an orchestra. Everywhere, in economic as well as in political life, the guiding principle is one of ruthless striving for success at the expense of one's fellow men. This competitive spirit prevails even in school and, destroying all feelings of human fraternity and cooperation, conceives of achievement not as derived from the love for productive and thoughtful work, but as springing from personal ambition and fear of rejection.
There are pessimists who hold that such a state of affairs is necessarily inherent in human nature; it is those who propound such views that are the enemies of true religion, for they imply thereby that religious teachings are Utopian ideals and unsuited to afford guidance in human affairs. The study of the social patterns in certain so-called primitive cultures, however, seems to have made it sufficiently evident that such a defeatist view is wholly unwarranted.

Source: Here on Earth

Oscar acceptance speech for his performance in American Beauty (March 2000)