What are some possible ways to invade a country with 15,000 square miles of desert in the way?

In my story, there are four countries, all with the same culture and customs, but they have different governments and leaders. Country A is a dictatorship, Country B is a democracy, Country C is a theocracy, and Country D is a constitutional monarchy.<br><br>Country B and Country D are right next to each other, and Country C is to the east of B, while Country A is west of D. Unfortunately, there is a 15,000 square mile desert in the middle of Country A and Country D, which is a big barrier between the two countries. Country A wants to invade Country D, but they would have to go through that desert or through Country B. <br><br>Here is a list of the leaders and their personalities:<br><br>Country A: Xarath, the Cruel<br><br>Country B: Elara Moonwhisper, the Wise<br><br>Country C: Arin the Second, the Righteous<br><br>Country D: Lila Earthhaven, the Bold<br><br> Country A has the most military power out of the four countries. I want Xarath to invade Country D, but I want them to lose, and I want Country D to actually end up taking Country C. I know this will require quite a bit of planning, and I want to make sure that the invasion is realistic. <br><br>Some things to consider:<br><br>Country A/Culture A is based on ancient Egypt, and they live in a world with magic. I've researched ancient Egyptian warfare, and I know that they were quite skilled at sieges. <br><br>Country B/Culture B is based on ancient Greece, and they live in the same world with magic. <br><br>Country C/Culture C is based on the Holy Roman Empire (but without the connection to the Roman Empire part), and, again, they live in the same world with magic. They worship a single male deity who is worshiped by all of Cultures A, B, and C.<br><br>Thank you for your help, and I hope that you enjoy my story when I release it!<br><br>Update: I changed the names of the leaders. Thank you to those of you who pointed out that they were a bit cringy sounding.