The simplest way would be to take a page - or an entire chapter - from Orson Scott Card's Ender universe. Search for "Demosthenes' hierarchy of foreignness" for more details; below is a rough overview.

Basically, there are degree of strangeness when it comes to other life. Aliens could fall into any category.

• There is the sort that we recognise as being just like us, but with a different culture. They might have a different language, but we can translate; they might have odd customs, but their basic morals and instincts are like ours. We can live with these creatures; it's no different from human foreigners.
• There is the sort we recognise as being like we would be, if we were from the same place. For example, perhaps their planet has very different conditions, so they have different base instincts - but ones that we can understand; ones that we'd have if we lived on their planet. We can live with these, once we adjust and find common ground.
• There is the sort that we recognise as being intelligent life, with whom we share some basic or important ideals. They may have a radically different worldview, or a totally different concept of what is moral. We can coexist with them as a species, but it may be difficult to live together. It will be hard to form a system of common ideas and concepts so that we can communicate, and it's unlikely that we'll be able to intuitively understand how they think, but it's possible.
• There is the sort that we recognise as being intelligent, but is too strange for us to comprehend. No matter how hard we try, we can't find enough common ground to form a basis of communication, and therefore we can't work out what they want. We can't coexist with them, because they could do anything - including kill us - and we can't tell them that we don't like it. Likewise, we could do things that they will not tolerate, and we have no way of knowing. We can avoid them, perhaps, but not live with them.

The general idea is that if your aliens are of the last sort, war may be unavoidable. If you can't find common ground with an alien race - if you can't communicate with them, even at the level of simply expressing ideas or emotions - then there is no way to negotiate. If they are sufficiently strange by human standards, then there can be no empathy or shared instinct between us, which means neither of us can predict how the other will act or react in a given scenario. They might not even realise that killing us is something we don't like, and without communication, we can't tell them. This makes them a threat, constantly, regardless of the situtation. If we can't live apart from them, and we can't keep them out, then our only remaining option is to destroy them.

Of course, once you resort to violence, you have to be sure that it'll work, particularly with an enemy that's unknowable, and doubly so if they have better technology or military power than you do. You need to remove their ability to respond as quickly as possible, or you're toast. Their motives may be strange to humans, but in general you can assume that any intelligent creature that hasn't been killed off yet must have some sort of survival instinct which includes the neutralising of threats, and you're about to make yourself a threat to them. If violence wasn't your last resort, you failed to resort to enough of it. In human conflicts, there's often a back-and-forth exchange, a slow escalation, with neither side wanting to overdo it for fear of doing too much damage to the land they'll have to live in afterward - or being seen as a dangerous aggressor by the international community. With an unknowable alien, you can't be sure that the rules of this game will be followed. They may meet your first strike with a retaliation by the full force of everything they have. Even if they don't, the longer the fight goes on, the more they wear you down, and the more chance they have to do something you weren't expecting. Therefore, you have to hit them as hard as you can in your first strike, to maximise your chance of success and minimise the uncertainties.