Ok, examples. Germans have strong tanks, bad aviation. When playing against a german you can either invest in AT, or AA at the very beginning. It is less likely he attempts an early paradrop on your base because he has a longer way to actually perform it and it will cripple is economy for a bit. It is much easier for him to try an early tank rush. So you deploy some AT at first.
Then you see some unidentified heavy units moving your direction. You ATs so far probably would be enough, but because he went apparently this way, you invest in more AT. Suddenly a plane comes flying and a paradrop inf takes you base, you loose. Your mistake was to be afraid of those heavy units, which turned out to be fake units anyway and you didn't invest in AA at all.
Next game: you invest some in AT, some in AA. But this time it turns out those heavy units were real, and more than your AT could handle. Your mistake? You didn't buy a single scout who could've found out in time what's what.
These are just beginner battles. Pro battles are much more involved series of calculated risks which you throw at each other, until one of the two makes a mistake, or doesn't go through with something he began (happened to me a few times where I initiated a tank rush, got held up by a few desparate AT, could've steamrolled them, but was afraid of what else he was cooking up in his base and so ultimately failed to attack).
Once I built some tanks, found him building lots of AT, let the tanks stand around, built a number of bombers, found he upgraded to lots of AA, then built some artillery, found that it couldn't do shit because I couldn't keep my scouts alive near his AT hidden in the woods and lost, because through my fuddled attempts of sniping him with the souts/arty combination lost track of time and missed the clues that were leading up to his sudden huge air bomber assault, which was way oversized for my meek afterthought-AA anyway. So in the end he won despite having earned and spent much less resources than me (he was kind of stuck in his base because of my tanks ruling the map).