Defence and defense are different spellings of the same word. Defense is preferred in American English, and defence is preferred in all other main varieties of English, including Australian, British, and Canadian English. The spelling distinction extends to most derivatives of defence/defense, including defences/defenses and defenceless/defenseless. But the words defensive, defensiveness, and defensively have an s everywhere.
Though defense is now the American spelling, it is not American in origin. The OED and Google Books reveal examples of the spelling from as long ago as the 1300s, many centuries before the United States existed. That spelling continued to appear a fraction of the time through the 19th century, when it was taken up by American writers. Today, to the chagrin of those who dislike American English, the spelling is gaining ground throughout the English-speaking world.
This ngram, which graphs the use of defence and defense in American English through the 20th century, shows that defense became the prevalent spelling around 1910:
And the next ngram graphs the occurrence of the words in British English during the same period. It shows defense gaining ground:
U.S. publications use defense—for example:
The Redskins are last in the NFL in defense, giving up an average of 394.8 yards per game. [Washington Post]
Schools in Southern Section Division 1AA had better start preparing for Etiwanda’s man-to-man defense. [Los Angeles Times]
A senior U.S. defense department official says China’s military buildup could turn the Asian regional security balance upside down. [Voice of America]
Non-U.S. publications use defence:
The Irving shipyard, for instance, has landed one mighty large contract from the defence-friendly Harper government. [Chronicle Herald (Canada)]
An assessment of civil defence organisations says Emergency Management Southland is among the best in the country. [Southland Times (New Zealand)]