"He was issued a helmet, ready to go," coach Rod Marinelli said Wednesday.
Now, Campbell, who was the Lions seventh-round draft pick, is closer to joining his fellow West Point graduates in Iraq or Afghanistan.
"When I got drafted, I told people that I was going to have the best of both worlds," Campbell said. "I was going to be in the United States Army and I was going to have a chance to play professional football. Now, I have the best of one world and I'm very positive about that. It's all going to work out.
"I'm in great shape and I'm going to stay in great shape. I'm going to fulfil my duty to the United States Army and do what I've got to do. One day, hopefully I'll get another opportunty to play in the NFL."
Campbell said he was initially upset when informed of the U.S. Army's plans.
"Oh yeah, I cried, because I wanted a shot," he told The Oakland Press of Pontiac for a story published Thursday.
The Army revised its interpretation of U.S. Department of Defense policy two weeks ago regarding soldiers playing professional sports, requiring cadets to complete two years of active duty before applying for a release. Campbell and the Lions didn't officially receive notice of the change until the eve of training camp.
"It's unfortunate, but it doesn't mean Caleb Campbell's dream is dead. It just means it will be delayed," Army spokeswoman Lt. Col. Anne Edgecomb told The Associated Press. "We want to take care of soldiers and dashing their hopes is not what we intend. But it is what it is."
Edgecomb said minor league baseball players Nick Hill and Milan Dinga, former West Point standouts, will be allowed to finish their seasons before eventually joining their units.
"We did an internal review of our policy and found that based on the DOD policy, we needed to adjust our policy," Edgecomb said Wednesday.
Campbell agreed to contract terms but did not sign the deal. The Lions will retain his rights until the 2009 draft, but he will not be eligible to play until 2010.
"Obviously, he's disappointed," said Marinelli, a Vietnam veteran. "But I obviously know what he's about. He got his orders and he's ready to report."
When Detroit drafted Campbell in April, it created a lot of publicity and led to some debate whether it was fair for a cadet to play pro sports while classmates were at war.
The buzz might have also made the Navy and Air Force bitter because their graduates were playing under different rules under the same Department of Defense directive, which was implemented in 1994, reiterated in 2007 and again just a few days after the NFL draft.
"The policy has not changed," Department of Defense spokeswoman Eileen Lainez said.
Navy Secretary Donald C. Winter ruled last month that Mitch Harris must serve a five-year active duty commitment. Harris, a 22-year-old pitcher with a 95-mph fastball, was selected by the St. Louis Cardinals in the 13th round in this year's draft.
Harris acknowledged being surprised by the ruling because Campbell was being allowed to pursue football while completing his military service as a recruiter and in the reserves.
"Army has redefined the Alternative Service Option to include playing professional sports," Navy athletic director Chet Gladchuk has said. "Our coaches are now operating under a significant handicap when recruiting head-to-head with Army. It may not be reflected on the playing field today, but I can guarantee you that it will result in a competitive disadvantage down the road."
The Air Force agreed, saying the academies recruit cadets from the same pool of candidates.
Last month, the Army embraced the advantage.
"The real advantage for the Army is just the amount of publicity we get," Edgecomb said in an AP story published on June 13. "When you think about it, who's the best recruiter for the Navy you can think of? David Robinson. He's called the Admiral, for goodness sake. The attention that we get in our primary demographic to have someone playing sports who's in the Army, that's where [we] in the Army see the advantage in this program."
Before he became a superstar center with the San Antonio Spurs, Robinson served two years of active duty for the Navy after graduating from the academy in the 1980s. He benefited from a policy that allowed him to apply for an early release to pursue "an activity with potential recruiting or public affairs benefit to the Navy and Marine Corps."
In 1986, Navy running back Napoleon McCallum played his rookie year with the Los Angeles Raiders while stationed at the Long Beach, Calif., naval base.
The Army changed its policy on July 8, but it wasn't until July 23 that the Lions received a letter from U.S. Army Lt. Col. Jonathan P. Liba, informing them in writing that Campbell had to cease playing football in order to perform "full time traditional military duties," until at least 2010.
"It's unfortunate that the timing of the new policy is happening at the same time that he was about to begin trying out, but that's not something we planned," Edgecomb said. "But he's been at West Point for four years and he went there to be an officer. What he's accomplished on a football field has been outstanding, but what he'll accomplish as a soldier will be even greater."
ESPN.com's Kevin Seifert and The Associated Press contributed to this report.