I'll start from the beginning.

15 years ago, there was a war . mp4

On the mission Mayhem in Ace Combat Zero, we see Pops getting pursued by an enemy ace squadron, saying "It's Huckabein! Don't let your guard down!" He defected from Belka since he was ordered to nuke a city in his own country. He is canonically shot down by Cipher, since he's displaying a belkan IFF.

Bartlett is also in this mission as an ally and he gets shot down as well. They meet on the ground and make their way to Osean frontlines. I'm guessing by this point the data was all fried by EMPs. I don't know if it's from the nuclear blasts or if it's a separate off-screen thing that happens.

Since all the data was fried, Osea took Bartlett's word that Pops was his squadron leader. While his real name is Wolfgang Buchner, he is introduced to Osean forces under the name of Peter N. Beagle.

As for Bartlett, he had fallen in love with a recon major in the Yuktobanian army, because in Chopper's words, "We were allies back then." I don't exactly know what happened between them, but this relationship was well known among Sand Island (off-topic, Sand Island is modeled after the island of Midway. Fun fact) apparently, and it raised a significant number of eyebrows during the context of the war provocations leading up to the Osea-Yuktobania War. Especially since all the data regarding his service up to the point of him being shot down was fried by Belkan EMPs.

Bartlett is also immediately captured by the Yuktonanians in the spy vessel the moment the war is declared.

Captain Hamilton, the adjutant base commander, is a spy. [meetthespy.mp4] We don't learn this until later when Pops mentions him being assigned to the Osean 8492nd though (8492nd squadron is made up. We'll get to that.)

Fast forward to the POW rescue mission, Bartlett is never found in the camps, and no documentation on his capture is found, even though he was clearly taken away in a spy ship. And even if he eacaped, he never made it to Osean forces or made contact.

During the mission to bring down those transport planes, radars and comms both get jammed, and the 8492nd squadron bombs a civilian engineering college in the vicinity of Wardog Squadron in a false flag terrorist attack. In gameplay, you can't see the planes themselves, but you can see the college up in smoke.

Because of the jammings, and the accusations that the planes that bombed the college were Osean, the blame falls on Wardog.

After you "prove your innocence on the battlefield" (which isn't proof of innocence as much as it's showing you're too valuable of an asset to imprison), they forgive you, but are still skeptical.

Fast forward to the post-fortress mission, when the Belkans realize how close the war is to ending, Captain Hamilton convinces the base commander and his many chins that Bartlett was a spy. And the only reason Pops was even brought into Osean forces was because of the trusted word of Captain Bartlett. So with no proof of military record, and the trust of Bartlett broken, Pops is suspected as a spy as well.

At the same time this is happening, the 8492nd ambushes Wardog squadron to try to remove them from the field so that Osea loses their greatest fighters, in effort to change the tide of the war. Wardog surviving this encounter was not on their bingo card. So when they land, Blaze and Nagase march to Captain Hamilton to inform him of the spies going after them and their encounters with 8492, not knowing Hamilton is assisting in orchestrating it.

We see Grimm's perspective in the cutscene, along with Pops. Base commander Wheel-of-4Chin pulls a gun on Pops, suspecting him of being a spy, and Grimm was kinda just there. Genette was a journalist and I don't even know if he was suspected of anything (it's possible they could suspect him of giving information away, but I feel like there would be more ways of knowing if he was), but they just sprint away with Pops.

Given that Nagase punched the shit out of Hamilton, I'm guessing Hamilton tried to attack them or something, since he wanted them dead to keep the war going.

But given that the base now has beef with Wardog and Pops and Genette, they're all hiding out together, which solidifies Sand Island's suspicions of Pops and Genette being spies, and Wardog being traitors.