Defence and defense are different spellings of the same word. Defense is preferred in American English, and defence is preferred in all other major varieties of English, including Australian, British, and Canadian English.
While the distinction extends to most derivatives of defence/defense—including defences/defenses and defenceless/defenseless—the words defensive, defensiveness, and defensively have an s in every variety of English.
Examples
For example, these U.S. publications use defense:
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]
And these non-U.S. publications use defence:
Japan’s defence policy is changing. [Financial Times (U.K.)]
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)]
Ngrams
This Ngram, which graphs the use of defence and defense in American English through the 20th century, shows that defense became the prevalent spelling early in the century:
And the next Ngram graphs the usage of the words in British English during the same period. It shows defense gaining ground:
