All seemed well between the NFL and the players’ union last July, when representatives of the two sides stood outside the union’s office in downtown Washington. They were there, ostensibly, to announce the completion of a 10-year labor deal but, in reality, mostly to congratulate each other for the hard work and long hours they’d put in to smooth over their considerable differences.

Owners spoke of their admiration for DeMaurice Smith, the union’s executive director, who initially rankled some on the management side after coming from outside the sport to take the job in 2009. Players talked glowingly about owners, particularly the New England Patriots’ Robert Kraft, who’d continued to toil at the bargaining table even amid the grief of his wife’s death. Indianapolis Colts center Jeff Saturday hugged Kraft, and both sides credited Smith and NFL Commissioner Roger Goodell for working in concert to push the deal through.

Now that moment seems like a long time ago. The league and the union have had a series of skirmishes since then that culminated Wednesday, when the union filed a collusion lawsuit against the NFL in federal court. The complaint accuses owners of conspiring to restrict player salaries in 2010, when there was no NFL salary cap, and estimates that players suffered $1 billion or more in damages.

That came on the heels of clashes over the suspensions of New Orleans Saints players for participating in what the NFL said was a program that paid them cash bonuses to injure opponents, and a vote by owners last week to require players to wear knee and thigh pads. The two sides also continue to battle over blood testing players for use of human growth hormone.

“Other than that, it’s been a pretty smooth offseason, right?” said David Carter, executive director of the sports business institute at the University of Southern California. “Whenever I see the reports of these kinds of things, I’m reminded that every development in and of itself is a subtle form of negotiation.”

Goodell, speaking Tuesday at a news conference in Atlanta at the conclusion of a one-day owners’ meeting there, called the persistent conflict “part of operating in a pretty complex world.

“You don’t expect to agree on everything,” he said that day. “That’s part of the dialogue. That’s part of finding solutions.”

Smith stood outside the union’s offices, on the same sidewalk where the labor agreement was hailed last summer, for a news conference Thursday. He called the NFL a “cartel” and said he was confident the union would prevail in its collusion complaint.

“It doesn’t have any sort of reflection or implication on friendships or anything else,” he said. This is a business. But when we believe that people work in concert to injure our players [financially], what would you have me do?”

Domonique Foxworth, the longtime NFL cornerback and the union’s president, said later: “Our relationship has been improving, but there are always going to be some issues where our interests aren’t aligned. And it’s just the way it is in a business like this. There are going to be some issues that, no matter what, are going to be contentious.”