阅读下文，回答相关问题。

                                                                                                       月夜忆舍弟
                                                                                                            杜甫
                                                                                            戍鼓断人行，边秋一雁声。
                                                                                            露从今夜白，月是故乡明。
                                                                                            有弟皆分散，无家问死生。
                                                                                            寄书长不达，况乃未休兵。

“（蒋）介石先生······领导全国各民族进行空前伟大的民族革命战争，凡我国人无不敬仰······***谨启。民国二十七年九月二十九日。”，这封信的直接背景是