Could South Africa intervene in Zimbabwe to rescue Robert Mugabe? Both countries train each other's military forces and, as James Kirchik points out in The New Republic,

As members of the SADC, South Africa and Zimbabwe are also signatories to that organization's Mutual Defense Pact. Article 7 of the agreement stipulates that "No action shall be taken to assist any State Party in terms of this Pact, save at the State Party's own request or with its consent." Thus, Mugabe can continue to run a police state and his neighbors can't do anything about it without his permission. Conversely, if Mugabe feels that the Movement for Democratic Change (MDC), his opposition, poses a threat, he could theoretically ask SADC members to help him stamp it out.