From the National Archives:
The Telegraph dramatically increased the speed with which military officers communicated with each other and with civilian authorities. Newspapers also used the telegraph and rapidly delivered war-related information to readers. Early in war however, the Lincoln administration censored telegraphic communication, arguing that while news of the war could be delivered with great speed, military secrets could also be quickly divulged. There was a fine line between military necessity and censoring the merely embarrassing or politically disadvantageous.