This official FRCP state document, heavily classified, lays out three primary stages of national defense against a vastly superior interstellar or inter-dimensional power.
Step One: Give up
No fruit would come of our armies, air forces, or navies. In open combat, our logistics would be overrun before the army itself knew what the status of their position was. So, it's better not to try.
Step Two: Guerilla Warfare
Remember all those armies, air forces, and navies? Yeah, they gave up fighting in the military manner, but they didn't turn themselves in. Therefore, attack helicopters would be hidden away in forests, tanks in the foothills of mountains, and fighters to offshore and far away assets and allies. Surface warships would be abandoned, but our prolific fleet of submarines would scatter.
The circumstances of resistance would constantly change, but common tactics on the ground would to be organizing criminal rings and resistance movements in urban areas, harassing soldiers and troop movements, and even booby-trapping buildings. Outside of cities, roads would be trapped and set up for ambushes, and most industrial operations would cease either by worker's strikes or uprisings. The boreal forests to the northern and southern borders would be used as shielding from prying eyes as well as well-known terrain for asymmetrical combat. Aircraft would likely be stored here, such as the VI-92 attack helicopter for short-range operations and firepower during larger operations.
Meanwhile, it'd likely be a cat and mouse game with the several nuclear submarines the FRCP has, notably the Lobanov Attack Submarine, capable of firing two ICBMs before reloading, and the infamous Yakov-Class Submarines, with 20 VLS built-in, with more missiles to be reloaded. They will provide much-needed firepower to the resistance, either to shoot down any transport ships descending from orbit or even to pick off any vessels hanging around LEO.
In dire circumstances, V̴̛͔̦͗̾̾́̍̒̄̂̊͒̎̈͆̇͘a̴̡̛̺͙̠̗̞̻̠͚̩̩̫̳̖͗̾́̊̆̄̀̈́̆̈͊̅̉͘͠ḑ̸̧̛̛̯̭̬̞̖̩͇̪̤̼̯̻̥̬̼̥̟̼̂͛̉̾̄͑̈̓̆̈́̉̇͜͜i̴̧̼̘̱̜̘̮͍͓͓̬͕̙̬̗͌̐̈́̌̉̾͑̓̀́̋̊̊͒̌͘͝ͅm̴͚̥͍̙̺̪̽̃͆̎͌̽̒͝͠͠ will be released.