Expression for the advantage of being in possession of disputed goods in a civil suite?

Some time (years?) ago I saw (In fact it might have been in a comic, possibly Zits.) an expression/proverb that basically said that being in possession of a disputed goods meant that a civil law suit/quarrel was almost won before it had begun. Anyone know what idiom am talking about and can remind me?

NB: I am not sure if it was in English or Latin. If possible I would prefer a latin answer.

Edit: with this answer I found the comic strip where I thought I read it first http://www.thecomicstrips.com/store/add.php?iid=90971

I also found a corresponding latin phrase, uti possidetis, ita possideatis (are there any better idioms in latin?), which is more useful since I am not gonna use this expression in an English context.

Solution 1:

The expression that comes to mind is

Possession is nine-tenths of the law.

From the Free Dictionary:

possession is nine-tenths of the law:

Custody presumes ownership. The basis of this legal maxim that comes down from the 17th-century is the commonsense observation that if you have control of something, chances are better than average that it's yours. Lawyers term it a rebuttable presumption: ownership is recognized unless disproved by someone holding a more valid claim. The phrase started life as “possession is nine points of the law,” which referred to possession's satisfying nine out of eleven factors that constituted absolute ownership. However, “nine-tenths” entered popular usage to reflect the idea that custody is 90 percent of legal ownership.

Solution 2:

Nouveau Petit Larousse: p.1125 (English paraphrase)

Beati possidentes
happy are those who possess
A phrase made fashionable by the prince de Bismark; it means if you want to claim ownership of a country you must take possession of it first.