SAN DIEGO, California (Reuters) - Presumptive Republican presidential nominee Mitt Romney chose Memorial Day to proclaim to the American people his conviction that the world is a dangerous place, and the United States must remain its most formidable military power.

"The world is not safe," Romney told veterans on Memorial Day. He was joined by Senator John McCain, in a speech to honor the veterans of America's wars.

The United States now has two paths forward, Romney said. He called one "the pathway to Europe," suggesting Europe had acquiesced to geopolitical threats. "To shrink our military smaller and smaller to pay for our social needs."

The other path, Romney said, is "to commit to preserve America as the strongest military in the world, second to none, with no comparable power anywhere in the world."

Romney, expected to face off against President Barack Obama in November, joined the 2008 Republican presidential nominee in thanking the nation's veterans.

Romney, who has focused his campaign on the struggling U.S. economy, changed his focus on Monday in his warning about the dangers of the world outside America's borders, indirectly criticizing Obama's foreign policies.

"I wish I could tell you that the world is a safe place. It's not," Romney said.

Romney ticked off Iran, Pakistan, China and Russia, among other countries, as threats as he transformed his message from economic warnings of the United States becoming like Europe to a military warning that America was becoming weaker.

McCain introduced Romney to the 5,000 people gathered as a "man who I believe is fully qualified to be commander-in-chief."

"He believes in American exceptionalism," McCain said. "He believes the 21st Century will also be an American century."

American exceptionalism is a political philosophy that assigns a unique and unprecedented place to the United States as the leading global proponent and exemplar of liberty, freedom and equality around the world.