What was Nixon's plan to "win" the Vietnam War? How did this plan involve Cambodia?
One of the portions of Nixon's plan to win the war not addressed in the previous answer was the plan to further strengthen the Republic of Vietnam's army, the ARVN, in order to allow the US troops to get out. This involved heavy investments in equipment and training for the Americans who were training and equipping the ARVN. Many US soldiers and leaders opposed this policy claiming that ARVN was not an effective fighting force lacking the morale and leadership necessary for combat operations.
The bombing of Cambodia was mainly intended to stop traffic on the Ho Chi Minh trail of which large portions were considered safe because they were outside the borders of Vietnam in Cambodia. Nixon was willing to allow incursions into Cambodian territory in order to try and interdict traffic bringing supplies and troops south.
President Nixon had a three parts to his plan to end the war.
First, he wanted the North Vietnamese to withdraw their military from the South.
Second, he wanted to end the kind of domestic unrest that was causing the protests against the war at home. He did this by making the draft fairer and eventually ending it.
Third, he wanted to fight the war more aggressively. This included bombing Cambodia because there were places in that country where communist troops would set up bases in safety (because the US would not attack them in a neutral country.)