The scenario takes places on a hypothetical planet, that is orbiting Alpha Centauri, but really could be just any generic planet. The place has an oxygen atmosphere that is breathable for humans, and a very well developed ecosystem, but no sentient life as of this moment. In terms of gravity and climate the dirtball is similar to Earth in most aspect, except for the fact that shoe areas are covered with certain meat-eating, floating organisms
Now - a colony ship from Earth has just finished its trip, the computer has thawed out the crew and they are getting ready to set up their new home. There are one thousand people on board, about a hundred of them are soldiers, chosen from the best and brightest special forces operatives from every country, and additionally trained extensively before the mission. The rest are scientists and engineers in about even proportions, plus a few computer experts and astronauts to operate the shuttles. They have food to supply the crew for a year, some prefabricated buildings, a shitload of lab equipment, light utility hevicles to be assembled on the spot, pre-fabbed oil rigs, some raw materials, in short - everything they need to build a first city, find and exploit raw materials. The soldiers are armed with light weaponry - mostly assault rifles, grenades, sniper railguns and some RPGs, just in case. The also have ten heavy machineguns that can be mounted on hevicles or tripods, also "just in case". Every crewmember has rudimetary training in survival skills and firearm usage in addition to their normal specialities. The ship is equipped with five short range shuttles, that can be used to ferry stuff to and from orbit, but fuel supply (hydrogen/oxygen) is very limited.
Now, your biggest assets are environmental suits that the entire crew is equipped with - they are bullet-resistant, yet comfortable to wear, they also provide some capabilities of powered armor, can be pressurised on a moments notice, and the neat fold-in helments posses air filters. Every suit has a built-in sensor suite with IR sensors, motion detectors, x200 zoom feature and scrambled radio. Soldier's suits are significantly tougher and, shall we say, "stronger" that civilian versions - they basically can stop rounds fired from their own rifles pretty good. The suits are also connected directly to the neural system of the wearer, and this allows soldiers to jack into their weapons, projecting a targeting reticle directly into the brain. The suits are powered with extremely miniaturized hydrogen fusion cells, that can be recharged using facilities on the starship, provided you've got enough hydrogen. The can work for about two weeks before needing a recharge.
Now for the case of the argument - two years before the Earth spaceship has arrived, a similarly-sized group of aliens have landed on the planet (their make-up is also similar - a hundred crack soldiers, scientists, technicians, computer experts). The aliens are three meters tall, amphibious (they can swim VERY well, even in their armor) and significantly stronger than humans. They also use plasma based weapons, giving them a lot of power in one rifle, but they lack machine guns because of that. They also posess powered armor technology, but it is bulky and needs to be recharged often, so most of their troops actually don't use powered armor, relying on their feet instead. Those aliens have already established two large bases on the surface, centered aroudn the oil deposits on the main continent, plus one underwater base on the continental shelf. They don't have a space ship of their own, they have cannibalized theirs a year after the landing, but they do posess atmospheric aircraft - they are lightly armed, but can easily traverse vast distances, so basically the aliens have global reach. Their craft are about the size of modern choppers, and are similarly armored, but they use gravitic drives or something similar. The green dudes know the environment well and are extremely hostile to anything that doesn't look like them and can use a weapon. Neither side has the ability to leave the planet and return to their home world. Both sides, however, posess one genetic lab containing stored gene samples that are absolutely essential if they want to have hope of starting a viable colony. The aliens store their samples in the underwater lab, the humans have theirs on board the starship.
The war starts when the aliens attack and destroy one of three camps that were established by the humans on the surface of the planet. This is also their first contact.
Now, let's suppose that you were colonel John McMillan (the NPC that is in charge of the mission from the human side). How would you go about winning a new home for the crew of your starship? You don't know anything about your enemy, except that they wear powered armor and use plasma weapons. The enemy doesn't know much more about you, too. How about from the alien side?
Now, let the fight begin!