It is possible to use cards from more than two nations during a match. This is not cheating, it is a distinctive feature of these cards, and following the game rules and game design.
There are a couple cards in the game that allow you to get cards from another nation during match time. Especially Atlantic Convoy and Night Raid are noteworthy for potentially providing cards from other nations.
The British order ATLANTIC CONVOY will add US units to the play regardless if you have or have not the US as major or ally faction in your deck.
Atlantic Convoy is a card of special rarity. This means you can have up to two of these orders, and therefore you can receive four US cards in total.
The British order NIGHT RAID copies a random order from your opponent's deck to your own hand regardless of the nation. As such, you can receive an order from a nation different from your major and ally nation.
Night Raid is a card of special rarity. This means you can play up to two of these orders and receive two cards from other nations.