Complex sentence: This did not escape the attention of the watchful Bruce; and, to show his appreciation for the great service, he assigned to Angus and his descendants, forever, the honorable position of the right flank of the royal army.
Split into simpler sentences: This did not escape the attention of the watchful Bruce. To show his appreciation for the great service, he assigned to Angus and his descendants, forever, the honorable position of the right flank of the royal army.