It is obvious that the Lamanites were able to maintain huge military forces on both the east and west fronts. This also compelled Moroni and Teancum to divide the Nephites on two fronts.
Moroni's predicament was the result of fast action by the new king Ammoron. We learn he did not stay in the city of Mulek with his troops but went back to Nephi to report to the queen and mobilize a new army to attack in the west where Moroni was located.