The legal battles between businesses and credit card companies have mainly seen Visa and MasterCard as the defendants. However, a trial started this week that has American Express defending itself from allegations from the Justice Department that it is trying to stop competition.

According to an article from Time Money, the battle has been ongoing for nearly four years. The Justice Department originally brought the suit against Visa, MasterCard and American Express in 2010. Visa and MasterCard quickly settled, but AmEx fought.

The allegations stem from American Express' "take it or leave it" policy. Merchants are required to pay a fee - generally between 2 percent and 3 percent of the total purchase - every time they process a credit transaction. The government claims American Express charged the highest merchant fees of any card network but also forbids any of its merchant partners from offering customer incentives to use cards that are cheaper to accept.

The justice department claims that this is anti-competitive. AmEx is responsible for 26 percent of all money spent through credit cards in the U.S., making it too important for businesses to turn their back on. This leads to higher prices across the board.

American Express countered by saying it is too small to have an anti-competitive effect on the market. On top of that, the higher fees are needed to provide other services like fraud reduction programs, financing and marketing and data analytics.

The outcome of this trial is going to have an impact on credit card processing and merchant interchange fees, though an outcome will likely not be known for a few months.