Complex sentence: But his excursion to Gettysburg, ostensibly to find shoes, essentially a reconnaissance in force conducted by a full infantry division, had indeed started a general engagement and he was on the losing side of it so far.
Split into simpler sentences: But his excursion to Gettysburg, ostensibly to find shoes, was essentially a reconnaissance in force conducted by a full infantry division. This indeed had started a general engagement and Heth was on the losing side so far.