As always in science fiction, science does come to the rescue, in the form of a German-speaking female scientist with a talent for interrogating captured aliens (perhaps she was recruited in South America?). Assuming she doesn't end up on trial in The Hague, her lab will develop a series of high-tech weapons and devices, mostly based on captured alien technology, that can be fabricated by XCOM's workshops. However, only one research project can be pursued at a time, so the XCOM commander must prioritize between lines of research such as better weapons, armor, and interceptors. Yet the real obstacle in the interplanetary arms race is that most advanced gear requires unearthly raw materials that can only be obtained from wrecked alien spacecraft and fallen alien warriors. Thus XCOM must make the war pay for itself: the more aliens it destroys, the more resources it captures to fuel its own capabilities.
The tactical portion of the game ensues when troops are dispatched to the site of an alien raid, or during a special mission such as retrieving a top scientist caught in an alien attack, or when a team is dispatched to the crash site of a downed alien spacecraft. Tactical brilliance has not tended to be a hallmark of SF aliens, as in the 1970 British series "UFO," where Earth defense force SHADO (which bears a remarkable resemblance to XCOM, except mercifully for the purple hair) maintained three space interceptors on the Moon, yet spacefaring aliens were too stupid to overwhelm the defenses by attacking with four UFOs simultaneously. No such luck in XCOM, though. The aliens will raid three cities simultaneously, and the sole XCOM Skyranger transport with its six-trooper combat team can only respond to one attack at a time.
Once on the ground, the aliens are formidable fighters. I learned more about basic infantry tactics from XCOM: Enemy Unknown than any other game I've played. One reason is that it's turn-based, so instead of the mob tactics found in shooter games, you have time to analyze the situation. And you had better analyze carefully, because the computer-controlled aliens are aggressive and smart. They will utilize cover and employ flanking tactics to get better shots at your troops.
Each human soldier gets two actions per turn, including move, shoot, reload, overwatch (which means shooting as the enemy moves), and various special actions such as suppressive fire. The most fascinating part of the tactical game is the use of colored symbols to indicate where there is cover, and more importantly, from what directions. That wall or parked car may shield your troops from fire from directly in front, but not at a 45-degree angle. Against aliens who aggressively outflank (or the rocket-propelled Floaters who love to drop behind you), success means not just finding cover, but the right cover. It also means patiently using fire-and-movement tactics, because the aliens will chew up a banzai charge. Combat continues until all the aliens or all the humans are wiped out, except for certain alien terror raids where the XCOM team must race to save as many civilians as possible.
It is the fate of XCOM to always be racing against time. When successful alien raids induce eight of the 16 XCOM Council members to withdraw, the game is lost. But like ballistic missile defense, no matter how capable the defenders, some of the attackers will get through. Thus, time is the enemy, and so is money. Over time XCOM will need more expensive weapons and facilities, but inevitably Council members will leave and take their funding with them, which means a shrinking resource base for Earth's defenders. Responding to alien raiders will not be enough. To win, XCOM must strike at the...sorry, no plot spoilers here.
Yet despite the specter of alien conquest, the scary creatures and burning cities, XCOM is a fundamentally optimistic game of the future. It offers an almost Star Trek-ian world where humanity puts aside its differences and works together toward a common goal. Yes, national self-interest does rear its ugly head. Nations will defect from the alliance if their individual defense is not assured, captive to an "all politics is local" philosophy that Gene Roddenberry would not have approved of, but is nonetheless a tradition of human existence.
Still, humanity does band together, if only hesitantly. The lion sleeps with the lamb, the Arab fights alongside the Israeli, the Chinese beside the Japanese. It took an alien invasion for this to happen. But if Earth does prevail in the war to save mankind, then perhaps the price was worth it.