In 338 BCE Philip entered the Peloponnese in force to finally force upon the Greeks his ultimate and long-sought goal, which was the unification of most of the Greek cities under a common peace (with himself as the hegemon) so that more grand plans could be put into play (namely an all-out attack on Persia). The goal of a nominally unified front of city-states meant that destroying a city, especially one as involved in the goings-on as Sparta, was not advisable. In 338 Sparta remained aloof, even though virtually everyone else was already in Philip's pocket (Corinth and Megara surrendered immediately; Argos and Elis were already his). Philip invaded the ancestral homeland of Sparta (Lakonia), but did not attack the "city" itself (Sparta was never really much of an urban center). I think this is the event you are talking about.

Why not? We are not told specifically, but we can surmise that he did not want to mar his supposedly unifying intentions with blood, especially the blood of a city-state with such a long and contentious history. The Spartans, even in 338, were a symbol. Philip was happy instead to simply contain them, without much bloodshed, and give most of their disputed territory to Sparta's neighbors (who were ancient, ancient enemies): Argos, Messenia, Tegea, Megalopolis. This hemmed in Sparta, left them isolated, and therefore unable to cause trouble. Sparta had been an invincible, fairy-tale force in the 6th and 5th century, but their utter defeat at the hands of the Thebans in 371 at Leuktra did much to dispel that legendary reputation.

In 337, delegates from pretty much every Greek city that mattered showed up at Corinth for the great summit--except Sparta. Philip's Common Peace was a brilliant step (especially after the failed attempts earlier in the century), but it was very clearly meant by Philip to appear voluntary. Thus Sparta was, alone, allowed to sit out and pout. Compelling her by force to participate would shatter the illusion of voluntary participation he was aiming to achieve.