I have tried to conceive the most realistic hard sci-fi human/alien conflict scenario without invoking FTL magic. By realistic, I mean “War is the wrong answer but we do it anyway”. Diplomacy has failed and “here we are.”
Two interstellar civilizations have had a long-standing trade relationship, exchanging goods with centuries of lead time using 2 billion metric ton cargo barges (like the Nostromo), and exchanging information in real time through a quantum entanglement communication link. (The fictional world observes the PT Symmetry theory of quantum mechanics which means the “no-signaling principle” is not needed or valid). As such the quantum device must be exactly in the center of the two receivers at each system. See diagram below:
The civilizations are 12 light years separated, which means the communications relay is 6 light years from either civilization. This took almost a century to set up (It's not easy). This is basically a follow-on to Rocheworld.
The original trade agreement was insufficient, things have changed for both sides, but both stubbornly refuse to give up their leverage. One side began with suppressing critical medical exchange information claiming “your shipments are not sufficient.” Thousands were permanently disabled or died as a result. The other responded with coercive tactics of their own. Your goal is to increase shipments from the other side due to a pending long-term climate crisis; they are refusing.
How can these trading societies have information warfare leverage over each-other?
Physical resources of value are centuries away from making any impact. Information warfare is the only tool available. An answer identifies ways to weaponize information assumed to be valuable to the other side. Please see full answer checklist below.
GEEK SECTION / TECHNICAL
Fastest space flight capability is 0.15c (1 billion mph) for either civilization
Cargo ships travel 0.1c on autopilot like Nostromo from Alien. Living pilots carry the outbound barges to the edge of their system, set the barge loose on course, then transfer to an inbound ship to pilot it home. (Barges are closely synchronized so pilots are not flying home empty)
The long lead time makes these resource transactions impractical for a free market. No one could take possession of the purchased goods, so the resources are traded via this pipeline which requires government oversight as a commodity with the product portioned out at the end (much like the US National Pipeline).
Stasis (cryogenic sleep) is not possible. One civilization is human, the other is alien with a 120% lifespan, but 75% of human constitution.
The climates are completely incompatible, which is why the resources are so valuable. We have things they can't (like liquid water), and vice versa.
Like our Internet, the link is vital both for civilian exchange as well as propaganda and conflict. Just as Isis would never destroy the Internet (how would they recruit or stream beheadings??), neither civilization wants to damage the link. Everyone loses - even the activists - if the link is broken.
The communications data rate is roughly 100Gbps - enough for 20 simultaneous 3D HD multi-angle video conferences (holograms). This link also exchanges valuable culture, scientific, educational, and entertainment resources unique to each world.
The relay is automated. It has a nuclear reactor, some stabilizing ion propulsion drives, and quantum entanglement laser emitters. Maintenance signals go back and forth twice per year but take 6 years to be received, then another 6 to be responded to.
ALL other communication happens at the speed of light. It takes years to communicate with anything between the planet’s, such as a barge or the relay.
Corpses can not fight. There can be no ground war or AI invasion, as nothing more than a microbe can survive the trip and sentient AI has not been developed.
Radical factions lack the resources to launch an attack on the relay (which would take decades with the best technology), but local receiving equipment could be sabotaged by wealthy factions.
Good answers have:
A cyberwarfare weapon which can coerce your position.
A plot device with Whatever information resource you choose to be weaponized - it will magically become valuable (but try to be reasonable here). E.g., "Planet A has an aquatic species which doesn't exist on Planet B. This species responds to an illness just like a problem on Planet B; A contributes medical research." "A demands more shipments, and 'delays' research findings until they get compliance"
Any support agency needed to make the weapon effective. For example, a religion (jihad), political (activist group tampering with an election), science (global warming council), diplomatic (United Nations), civil (labor unions) can exist, to give the weapon a punch.
An assumption of real-world (not utopian / Star Trek) ideologies. Do not try to "avert the war" or "solve the crisis" in the answer. All wars are fundamentally stupid, but a "let's all get along" mentality is fantasy. Don't challenge the premise of the war in an answer, just win the battle... ACCEPTING that it risks losing the war.
No suicide devices. Killing everyone is not a win.
Example from Star Trek: Voyager:
In Prime Factors the Sikarians highly valued “stories” as a commodity. Withholding or tainting these could be used as leverage, sending low quality or quantity “stories” until the other side bends.
(Note: I don’t like many Star Trek plot devices, I find them campy)
I will read comments if this needs expanding, please let me know before you answer.