Strategy is the practice of organizing armies and moving them on a grand scale to achieve advantageous tactical situations. It can also refer to the making and breaking of alliances to achieve a commander's overall goal.
Strategy goes hand-in-hand with logistics, which is the practice of organizing supplies for armies in order to achieve strategic success.
Strategy in the SCA
Strategy, in the sense of maneuvering large armies great distances, does not really come into play in SCA battles. Gathering and moving armies for an SCA war is very different from the how it would have been done in period, since the average medieval force couldn't drive to the battlefield confident in the quality of the roads, the reliability of communications, and an immunity to ambush while en route.