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.”

Even now, amid the combativeness, the two sides have nine more years of labor peace under the CBA and both parties are thriving. The salary cap system gives players about half of annual league revenue, which now tops $9 billion and could more than double by the time the current labor deal expires in nine years.

Carter said he believes the labor deal favors the owners. The union, he said, is going to great lengths to demonstrate its vigilance in protecting players’ interests.

“It reminds me of when the unions in other sports seem to feel the need to bargain and put up a barricade for the good of the players even if, for the good of the game, that particular thing might need to get done,” Carter said. He believes the union will have a difficult time proving collusion.

The union maintains that the labor deal is favorable to the players, and that it is always vigilant in advocating on the players’ behalf.

Gabriel Feldman, the director of the sports law program at Tulane University, said he doesn’t believe the league or union has changed anything about its usual approach. What has changed, Feldman said, are the circumstances. Last summer’s labor deal was reached under intense time pressure so no games would be canceled.

“I don’t know that it’s shocking that additional legal battles have come out of this. . . . I don’t read anything overly sinister into that other than each side is doing its job under the circumstances,” he said.

Even before the collusion lawsuit was filed, tensions between the two sides were running relatively high. The league and union agreed in their 2011 labor deal to blood-test for HGH, and targeted the start of the 2011 season for the testing to begin. But as the 2012 season approaches, the two sides still haven’t agreed on the details of a program.

The NFL and the union also are at odds over the NFL’s suspensions of four players in connection with the Saints’ bounty program. The union has filed a pair of grievances challenging the suspensions. Saints linebacker Jonathan Vilma has appealed his season-long suspension and filed a defamation lawsuit against Goodell.

The owners voted Tuesday in Atlanta to require players to wear thigh and knee pads during games beginning in the 2013 season. The league called the move a sensible and reasonable safety measure. The union suggested that the decision must be collectively bargained. Smith said Thursday the sport has more pressing injury concerns.

The collusion lawsuit was filed a day after the owners’ meeting, and a day after an arbitrator dismissed a case filed by the Washington Redskins and Dallas Cowboys contesting salary cap cuts imposed by the league in March.

The union had agreed to those reductions — $36 million over two years for the Redskins and $10 million over the same period for the Cowboys — and was named, along with the league, in the two teams’ bid for arbitration.

But then the union turned around and filed a case of its own based on the same set of circumstances. It accuses the league of having a secret salary cap of $123 million per team during the uncapped year.

Union representatives have said they agreed to the salary cap cuts for the Redskins and Cowboys in March because they didn’t suspect collusion at the time. They also said the NFL gave them little time to consider a take-it-or-leave-it offer that, if rejected, would have resulted a reduced salary cap for all 32 NFL teams this season.

The league has denied the allegations and maintains that the union is barred from suing.

The league also has had tussles, legal and otherwise, with players and former players that do not involve the union. Retired players have filed dozens of concussion-related lawsuits accusing the NFL that accuse teams of failing to protect them against the effects of head injuries. The league has denied those allegations.

Some current NFL players have publicly criticized Goodell for the amount of power that he wields, especially in the way that he and the league have handed out escalating fines for illegal hits. The league has responded that it regards player safety as a top priority and is safeguarding players, whether they agree with the approach or not.