Final answer:
Kenny O'Donnell instructs the pilot not to cross the Cuban airspace boundary to avoid provoking a military response and escalating tensions during the Cuban Missile Crisis.
Step-by-step explanation:
In the context of the Cuban Missile Crisis, Kenny O'Donnell (Kevin Costner) tells the pilot that he must not cross the Cuban airspace boundary. The reason for this instruction is to prevent any act that might provoke a military response from Cuba or escalate tensions further, potentially leading to a military confrontation or nuclear war. O'Donnell's cautionary advice reflects the sensitive nature of the reconnaissance missions and the broader strategy of President Kennedy to carefully manage the crisis without triggering a conflict.