RENO, Nev. — Their goodwill moment gone, President Obama and challenger Mitt Romney plunged back into their acrimonious political campaign Monday, Obama doubting Romney’s readiness to be commander in chief, Romney accusing the president’s team of offering “almost all attack ads.”

Days after the Colorado movie massacre brought reflection and talk of national unity from both camps, the fight was on again.

Foreign affairs made a rare move to the fore of the campaign as Republican Romney, the former Massachusetts governor, readied for a closely watched trip overseas. Obama, meanwhile, told a military audience that he was the only one in the race with a record, not just words, on international matters as he sought to undercut Romney’s travels before they began.

Both White House contenders are trying to gain the military vote. In the 2008 election, 54 percent of those who said they had served in the military voted for Sen. John McCain, himself a veteran, to 44 percent for Obama, according to exit polls.

Obama and Romney saw little time to waste in a tight, bruising race — and saw little need to apologize for any tactics.

A positive campaign “really would be nice,” Romney said even as he declared that sentiment over in an interview with CNBC. He blamed Obama for the tenor.

Obama told the Veterans of Foreign Wars he has kept his promises to end the war in Iraq, wind down the conflict in Afghanistan and go after al-Qaida leader Osama bin Laden wherever he was hiding. The terrorist mastermind was killed in a raid on Obama’s orders.

In every example, Obama poked at Romney without naming him. On Iraq, Obama suggested Romney would have kept forces in the war zone indefinitely.

“When you’re commander in chief, you owe the troops a plan,” Obama said. On Afghanistan, Obama needled Romney for opposing the 2014 timeline for ending the war. “You know what? That’s not a plan for America’s security,” he said.

Romney was to get his say before the VFW, too, today before setting out for England, Israel and Poland.

Trying to set the expectations for the opponent, Obama campaign officials challenged Romney to offer clear policy ideas during his three-country trip. Romney’s travels will be viewed as a measure of how well he can stand up on the world stage. Obama took an even broader such trip as a candidate in 2008.

Four years later, Obama said Monday: “We’re leading around the world. There’s more confidence in our leadership. We see it everywhere we go.”

A Romney spokesman, Ryan Williams, countered that Obama had “diminished our moral authority” in the world.

Romney noted that Obama aide David Axelrod had begun the day reminding via Twitter of the campaign’s request to see more of Romney’s tax returns.

“I haven’t seen the healthy, important debate coming from the president’s team,” Romney said. “It’s been almost all attack ads on all sorts of peripheral issues.”