Until very recently it seems like the history of much of the world was made up of the conquering and re-conquering of territory between vast empires and feudal warlords. During certain periods like the medieval era, a province could change hands a dozen times over without too much difficulty.
In the modern era as the current political landscape shows us, it is almost impossible for even the largest and most powerful nations on earth to hold on to even small territories. from Vietnam to Iraq to Afghanistan(twice) and now Ukraine, it seems like no one can grab even a sliver of territory for any length of time. why?
While we wait for an in-depth answer, it would be helpful to consider what exactly you mean by "conquest". Territorial annexation and cultural assimilation are not the same as demanding tribute or levies. Marching an army into foreign lands isn't the same as installing a functioning government apparatus. You also need to consider the differences in what it means to be "conquered" for a modern state versus historical fiefs, tribes, etc.