The first time he was offered a bribe by a friend and former mentor, Robert Porter says, he was “caught off-guard.”

The then-colonel in the Army National Guard adjusted to what he came to see as the “norm,” he said in a letter to a federal judge. For six years, Porter was involved in taking and facilitating bribes for recruiting and marketing contracts, part of a sprawling, multimillion-dollar web of bribe-taking that Sen. Claire McCaskill (D-Mo.) called in a 2014 hearing a “stain” on the Guard.

When he was approached by the FBI in November 2013, Porter quickly confessed. Not only did he tell the agents everything he knew, he recorded conversations with co-conspirators and testified against several. He pleaded guilty to conspiracy to commit bribery and bribery of a public official.

On Friday, Porter was sentenced in federal court in Alexandria to a year and a day in prison.

Several other people involved in bribery schemes involving the National Guard also pleaded guilty. Another, who Porter had taped giving a bribe, killed himself the night before his trial began. In a letter Porter received after his death, the former contractor said his longtime friend “would have to live with those consequences.”

Yet four former Guardsmen Porter testified against at trial went free after arguing that their payments to Porter were not bribes but bonuses. Porter had not collected the payments until he left the Guard and began working at their firm.

In a letter to the court, Porter, of Columbia, Md., said he did not regret his cooperation.

“I believe accepting responsibility was the honorable thing to do, and I believe these others know that and always will,” Porter wrote.

He said the bribes started as lunches, then dinners, and finally offers of money, all coming from his former military bosses and colleagues. He also said he believed the contracting firms did good work, so he justified his actions by saying no harm had been done.

Now, he says, he realizes he “allowed an unfair playing field to remain that way.”

William Cowden, an attorney for Porter, said the sentence was “fair and appropriate.”

Porter was on track to become a general when he retired to work for the contractors he ultimately testified against. He was a highly decorated veteran and Boy Scout leader who organized charity drives at work and did free home repairs for his neighbors.

He was allowed to begin serving his sentence next January, so he can see his older son through his last semester playing high school football and his younger son begin middle school.

“The hardest thing in this process was not collecting evidence against others,” he added in his letter, “but sitting down with my wife and boys to explain that their husband and father is an idiot.”