I don't really think that war is a plausible scenario in this case.
1. It takes unbelievably vast amounts of energy to travel interstellar distances. According to my calculations, to get an aircraft carrier (100,000 ton) sized spaceship up to 0.1c, you need 12577118.204822823 terawatt hours of energy, not counting efficiency (probably below 50%, not a physicist), which is over 600 times more than the combined annual electricity usage of all of modern humanity. Also, you would then need that amount of energy again to decelerate. While this could be done using fusion/antimatter engines or giant solar powered lasers pushing on sailships, it would be extremely expensive and impractical. Solar systems are full of tons of resources, with metal available in asteroids, planets, and moons, water in comets, hydrogen from water or in gas giants, so transporting inorganics at those distances is sort of pointless. Organics stuff can be grown in greenhouses at a fraction of the cost (remember, over a thousand times the entire modern world's energy budget just to move an aircraft carrier sized ship). Anything you don't have here can be made by starlifting hydrogen from the star and synthesizing it into whatever you need via fusion (might take energy to fuse together, but will get you what you need). Also, at 0.1c, it would take over 120 years (ships would be traveling slower at beginning when accelerating and at end when decelerating) for a ship to make the trip. Unless one or both sides has some sort of magical unobtainium the other side lacks, interstellar trade is pointless, because everything could be sourced much more easily from one's own solar system at a tiny fraction of the cost.
2. Any civilization capable of interstellar travel would be incomprehensible and perhaps even Godlike to our eyes. If you are comfortably capable of moving between the stars, your society probably has an economy billions of times larger than our modern one, at the very least. With much less technology than needed for interstellar travel, you would be able to fill your solar system with O'Neill cylinders and colonize every planet and moon available, so your planet of origin is no longer that important. 99
You will be able to stop almost every threat that presents itself, and to evacuate from most other things. You talk about global warming as a possible threat, but to an interstellar civilization, it would be nothing. They could easily stop it either by placing numerous mirrors around their planet to block sunlight or by using fusion powered plants to such CO2 from the air directly. Such measures would be expensive and impractical for us, but if they have the energy to travel at relativistic speeds, these things would be trivial.
3. Invasions are sort of impractical. Firstly, it would take decades for your ships to arrive, and when they did, they would be outdated (might be able to upgrade them en route based on new information, but not to latest level, probably) and outnumbered. The enemies would have had decades to build up defenses and your ships would be alone. With the resources available to them, they would probably be able to fill their Oort Clouds, Kuiper Belts, and planetary orbits with billions or trillions of self replicating autonomous defense platforms, while building reinforced bunkers on all their planets and placing guns on their planets larger than your ships. A better strategy would be to launch RKKVs or to burn your enemies with a Nicoll-Dyson beam, but while those might be very effective against a planetbound civilization, an interstellar one will have a presence across the system and would survive to retaliate.
4. Cyberwarfare is probably impractical. You might be able to hack the relay, but what would that give you? Turning it off would hurt you as well. The relay will probably be routed through a data center or something on the other side. If you try to hack them, they could just pull the plug on the thing, given that all your information has to go through that one route. 898