"Britain may be a small island, but I would challenge anyone to find a country with a prouder history, a bigger heart or greater resilience," Cameron tweeted.
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He followed up with another message two minutes later. "We are very proud of everything we do as a small island—and I definitely can't fit it all in a tweet."
He took to Google Plus to express his feelings at length.
"Britain is an island that helped to abolish slavery, that has invented most of the things worth inventing, including every sport currently played around the world, that still today is responsible for art, literature and music that delights the entire world. …" Cameron wrote in a six-paragraph post.
He listed the country's achievements, which included the world's sixth-largest economy, the fourth-best-funded military, "one of the best records" for art and literature, and contribution to philosophy and global civilization. He also mentioned Britain's role in World War II in helping rid the world of the threat of fascism.
Cameron's message met with a mixed response on Twitter and Google Plus. While many followers praised his patriotism, others said the U.K. was shamed by its failure to commit to a strike on Syria.
Syed Anwaar ul Haq Shah said, "The role you are playing is not good at international level. ... The message Assad got from the international community is that he can kill people."
(Read more: Cameron's fracking backing gets warm welcome)
Other followers took a wider look at Britain's role on the geopolitical stage.
"Yes, we have a proud history, but we have an awful lot of history which we shouldn't be proud of. … Yes, we helped to abolish slavery, but we were largely responsible for creating the transatlantic slave trade in the first place," George Forth said.