Final answer:
The additional K9 units sent to Vietnam were a response to the complex challenges US forces faced during the war, including the inefficiency of the South Vietnamese army and the need for specialized support in combat situations.
Step-by-step explanation:
The additional deployment of K9 units to Vietnam was a result of the escalating war and the challenges faced by US forces in combating the Viet Cong. The ineffectiveness of the South Vietnamese army, which was the primary ally of the US troops, led to a situation where the South Vietnamese soldiers, often unwilling to engage with the enemy, resorted to 'search and evade' instead of the intended 'search and destroy' missions. The US troops encountered difficulties such as fighting in an unfamiliar environment, dealing with a resentful local population, and allied forces that often seemed reluctant or inept. Moreover, the efforts to win the 'hearts and minds' of the Vietnamese people frequently backfired, adding to the complexities of the war.
These challenges and the unsatisfactory performance of allied South Vietnamese forces made it necessary to bolster the US presence with additional military assets, including K9 units. These dogs were used for a variety of tasks such as tracking, detection, and guard duties, and proved to be valuable assets in the difficult terrain and combat conditions of Vietnam.
Ultimately, the war's brutality and the US's inability to embed themselves successfully within Vietnamese society or support a stable and effective South Vietnamese government led to severe consequences. These included war crimes, a massive number of refugees, internal displacement, and a long-lasting impact on US military personnel in the form of PTSD and social ostracization.