Tour de force is French for feat of strength. In English, we use it to describe a particularly impressive display of skill and effort. The phrase came to English in the early 19th century, and it has become increasingly common ever since. Today, we use it not only as a noun phrase but also as a phrasal adjective.
There is no reason to hyphenate tour de force when it’s a noun phrase (e.g., her performance was a tour de force), but hyphenating it makes sense when it’s a phrasal adjective (she gave a tour-de-force performance). But it often goes unhyphenated even as a phrasal adjective.
We tend to italicize words and phrases from other languages when they are new to English, but tour de force is no longer new, so there’s no need to italicize it in normal use. But because tour and force are English words, some publications still italicize it to avoid confusion.
In French, the plural of tour de force is tours de force. In English, both tours de force and tour de forces are used (the former about twice as often as the latter).
Examples
Lord Byron thought proper, as a sort of tour de force, to versify, in his Don Juan, passages taken from prose works. [Fraser’s Magazine for Town and Country (1839)]
To pass this examination is a tour de force of which very few men are capable, owing to the general inability of the average mortal to take in abstract conceptions. [Music: a monthly magazine … (1895)]
At the same time, the trans-Atlantic flight is still a tour de force, as it will possibly be a very long time before we can hope to see an America-to-Ireland service. [New York Times (1919)]
It was an organizational tour de force. The French overlooked nothing, except the spirit of the Olympics with its emphasis on brotherhood and fellowship. [Calgary Herald (1968)]
Despite his late dismissal, Ponting allowed himself a touch of satisfaction as he looked back on his tour-de-force innings. [Daily Mail (2005)]
Nigel Hawthorne, best known as Sir Humphrey Appleby, delivers a tour de force performance as the king. [The Australian (2012)]