Initial Deployment (UFOAI)
At the start of a mission, your soldiers will be positioned inside or near the dropship. Any aliens which spot you on their first turn have all of their TUs, so they are able to execute a lot of Reaction Fire. Where possible, you should try to move into cover quickly. In many maps, though, your soldiers will be too far from cover to move there safely in one turn. To counter this, you'll need to find ways to disable the enemy threat so that you can take it out safely.
Throwing smoke grenades is the most common technique to handle the initial deployment risks. Throw a grenade just outside the entrance of the dropship so that it covers your soldiers. You can then use Scouts to move in and out of the smoke to spot threatening aliens nearby. Once spotted, place your shooters inside the smoke to prevent Reaction Fire and kill the aliens from there.
Be careful with the smoke. If your soldiers are all inside the smoke, they will not see approaching aliens until they are immediately adjacent. If there are several nearby aliens and no available cover, you may want to retreat to the dropship, using good Reaction Fire troops to guard the entrance. Smoke is most effective when it can be used temporarily to get your soldiers into better cover. This is not always possible, though, because many maps offer little to no cover for your troops.
If you only face a threat from one very close alien, you can try to disable the alien's Reaction Fire by throwing a flashbang in front of it. Then you can take it out safely without blocking your own field of view with a smoke grenade.