
Drill Sergeant Sullivan received an unexpected letter from a man named W in Netflix's Boots Episode 3, and it had a deeper meaning than fans realize. The comedy-drama series introduced Max Parker's Sergeant Liam Sullivan into the fold as an elite Recon Marine who may have a dark past. The high-ranking officers have high regard for Sullivan because he has field experience in various top-secret missions. Sullivan saw Cameron as a young version of himself (because both are still closeted), so he pushed him hard in the boot camp.
The mystery surrounding Sullivan was further highlighted when he came across a letter that shed some light on his past in Guam. It turned out that he was discharged in his previous mission from Alpha Harbor, Guam, for an unknown reason, but the letter he received gave a strong hint of what happened.
What Did the Letter Say & Cause In 'Boots'?

Boots Episode 3 ended with Sullivan receiving a mysterious letter from a man named "W," and he was seemingly rattled by it because he burned it down right after reading it.
Sullivan’s letter mentioned that something bad had happened in Sullivan's past mission in Guam, and NCIS (Naval Criminal Investigative Service) was investigating it.
"Looks like you left Guam just in time. NCIS is poking around, watch your six." - W
Episode 4 pulled back the curtain on why a decorated Marine like Sullivan is relegated to being a drill sergeant. It was revealed that Sullivan was discharged in Guam and needs to reenlist to stay in the Marine Corps. Captain Fajardo wanted him to reenlist as soon as possible because she knows what he's capable of.

In Boots Episode 5, NCIS agent Blodgett arrived to interrogate Sergeant Sullivan about his past in Guam, and viewers learned that the "W" in the letter was actually Major Aaron Wilkinson, an officer being investigated for misconduct and sodomy.
While Sullivan denied any romantic involvement with Wilkinson, it was clear that he is lying and there is more to their professional dynamic as two Marines. Blodgett further pushed the connection by requesting Sullivan to reveal a "Semper Fi" tattoo over his heart, the same one that Wilkinson has in his body.
While Cameron Cope is front and center in Netflix’s Boots, Sullivan‘s exploration of his own coming out journey is just as important, considering he is burdened by his past and it comes back to bite him at an unexpected time.
What Actually Happened to Sullivan? His Forbidden Relationship & Backstory Explained

Boots Episode 6 finally revealed what actually happened to Sullivan during his time in Guam with Major Aaron Wilkinson. It confirmed what fans suspected: he had fallen in love with Wilkinson, and they actually met off base and even exchanged "I love you" with one another. This also explained the matching "Semper Fi" tattoos over their hearts.
Wilkinson wanted to explore a life for both of them “after the Corps,” but Sullivan hesitated because no one would accept their union. One of his friends, Maitra, warned him that rumors are spreading about Wilkinson’s sexuality and he should avoid seeing him to avoid being linked to him.
As a result, Sullivan decided to get discharged from Guam and frame Wilkinson’s love for him as abuse. Even though he clearly loved Wilkinson, this pivotal move was Sullivan’s way of saving his career, but he clearly had regrets because that was his chance of finally coming out as a gay man.
Sullivan‘s scarred life and being afraid of coming out allowed him to sympathize with Cope because he knew that he was also a closeted gay man. Cope is in his phase of hiding and exploring his sexuality, which is why Sullivan wanted to help him deal with it and not be like him, who had deep regrets about life.

Speaking with Out, Max Parker admitted that Sullivan in Boots is "basically feeling guilty about maybe implicating his partner" and his "ultimate fear" is coming out because the society he's currently in is not that accepting of a closeted gay man:
"I think it’s quite clever in the show how it goes back and shows a bit more, and then you get to understand Sullivan a bit better. But I think, first and foremost, I wanted to make him… He’s a tortured man, so his drilling style was being more mysterious, and explosive. Knowing the backstory of Sullivan fed into the way he does everything. He’s under this pressure cooker, running away from basically feeling guilty about maybe implicating his partner. He’s running away from going to prison, too. And then, when it all starts to finally catch up with him, it’s this ultimate fear. That’s why he’s so explosive, and turns to alcohol, and turns to violence."
While Sullivan was in the trenches due to the NCIS' investigation of his sexuality amid his betrayal of Wilkinson's trust, it turned out that his friend, Maitra, was the one who actually reported Wilkinson. Although he couldn't correct his past mistake anymore, Sullivan found unexpected ally in the form of Captain Fajardo.
Fajardo outright pointed out that Sullivan's sexuality is none of the military's business, noting that she cared more about whether someone can do the job than their own feelings and preferences. The fact that Fajardo is the first female captain further cemented why she sided with Sullivan, because she knew the amount of discrimination one receives in their line of work.
Despite Fajardo's support, Sullivan decided to carve his own path and find a missing recruit named Jones. Alongside Cope (who joined him in the rescue op), they eventually found Jones, but Sullivan decided not to go with them and went off the grid by walking into the unknown.
Sullivan told Cope that he had finished his job by making him into a Marine before he disappeared into the night. At the time of Boots (which was set in the 1990s), being gay and going off grid was a crime under the Uniform Code of Military Justice, so Sullivan took a risk by disappearing despite his captain's support.
Boots' ending didn't really reveal what happened to Sullivan (unless a Season 2 is in the works), but he had a lot to deal with once he decided to come out of hiding.
Read more about what happened to Ochoa and how he died in Boots Episode 6.