Complex sentence: In late 1965 with the buildup of US airpower in South Vietnam the existing air bases were becoming overcrowded, in September plans to build an air base at Qui Nhon were suspended when the site conditions were found to be unsuitable.
Split into simpler sentences: In late 1965, with the buildup of US airpower in South Vietnam, the existing air bases were becoming overcrowded. In September, plans to build an air base at Qui Nhon were suspended when the site conditions were found to be unsuitable.