Have the military target any rioting or rebellion with extreme lethal force. Roll in the tanks and similar war machines when necessary. Begin an aggressive propaganda campaign demonizing the religious rebels, blaming them for the government assassinations and state of martial law. Use this public sentiment against the rebels to foster a sense of unity and nationalism, in order to perform heavy military recruiting.
As the new head of education, enact policies which will ensure students are taught the most fundamental principals of Sen, which is allegiance to the nation and its one true leader. Start this training in the very first grade and continue it to the last grade, whereupon they shall enter into mandatory military service with pride and honor.
Once things have settled down in however many decades, and the people of Sen are a massive population of military-trained atheist nationalists, begin focusing on opening trade channels with other nations for the purpose of economic development. Use the funding to build your military even further, increasing your standing army's size, training, and technology. Make them the most formidable force on the planet, even more so than all other armies combined, by many magnitudes.
Maintain a tight national border, and increase security to prevent unwanted persons from entering. Torture any spies discovered for whatever information they're worth. While spending money on the military, ensure that there are many surface-to-air turrets placed around the country, especially near the borders and major cities.
Also secretly invest in nuclear weapon development, and increase your nuclear weapons over time until you have enough to wipe out the rest of humanity a few times over. Have nuclear missiles aiming at every other place on the planet, and then unveil the information to the world, indicating that the missiles will be fired to wipe out all of humanity if any nuclear missiles are launched at them.
Then use your military to invade every other nation, and take them all over, becoming the ruler of the world.
