Complex sentence: Instead the battle was probably the closest the Australians came to defeat in a major battle in Vietnam with the Viet Cong had withstanding numerous Australian assaults as well as heavy artillery, mortar and aerial bombardment.
Split into simpler sentences: Instead the battle was probably the closest the Australians came to defeat in a major battle in Vietnam. To be sure the Viet Cong had proven themselve able to withstand numerous Australian assaults, as well as heavy artillery, mortar and aerial bombardment.