Manga about a boy whose girlfriend was drafted into war against aliens

Question

I think this manga is either a one-shot or a single volume. I read it in the early 2010s but believe it was written in the 2000s or earlier. I may be confusing it with another work in the same vein written around the same time with the same art style, but I think the cover art is of a teenage boy facing a teenage girl in a bombed-out urban environment, where the girl being pulled up by robot wings sprouting out of her back.

The premise is that Earth is being attacked by aliens, and the girl has been drafted into the Japanese army. The protagonists are high school students -- I think the girl is drafted because she fulfills a genetic/biological requirement for interfacing with a secret weapon. The boy promises to wait for her and sends her text messages every day as they are accustomed -- but as the front moves further into space, the messages take longer to get to the boy, and he has to wait months and years to receive each message.

I don't recall whether faster-than-light travel is involved, but I would guess that it is (that the girl remains a teenager while the boy grows up and moves on). The "camera" dwells on the fact that the boy and girl are using 2000s-era Japanese flip phones.

Answer 1

This is the manga adaptation of Makoto Shinkai's 2002 OVA Voices of a Distant Star.

A schoolgirl named Mikako Nagamine [...] is recruited into the UN Space Army to fight in a war against a group of aliens called the Tarsians—named after the Tharsis region of Mars where they were first encountered. [...] When the Lysithea leaves Earth to search for the Tarsians, Mikako's friend Noboru Terao [...] remains on Earth. The two continue to communicate across space using the e-mail facilities on their mobile telephones. As the Lysithea travels deeper into space, messages take increasingly longer to reach Noboru on Earth, and the time-lag of their correspondence eventually spans years.

Answer 2

I think the plot probably fits the answer of aethyr, the possible confusion might be with:

Saikano

I've only seen the anime, but there a girl is also turned into the strongest weapon and depicted with robotic wings from her back. But the attack is not performed by aliens and neither is she going into space:

One day, while Shuji is shopping in Sapporo, unknown bombers attack the city in broad daylight. He and his friends run for cover, but notice a fast and small flying object shooting down the enemy bombers. Separated from his friends, Shuji wanders through the wreckage—only to stumble upon Chise; here she has metal wings and weapons—apparently grafted onto her body. She tells him she has become the ultimate weapon, without her knowledge or consent, and that she is seen by the Japan Self-Defense Forces (JSDF) as the last hope for defending Japan. In the anime, it is not apparent why Chise was chosen to be the ultimate weapon or why the country is under attack. It was not until the OVA episodes were released that an explanation for Chise being chosen was offered: her body has the highest degree of compatibility with the weapon system.