History has numerous examples for the defenders being victorious.you don't win a conventional war with your forces in defenseSo? Why not?you don't win with your forces in defenseNorth Korea as created large tunnel networks (they were the architects of the tunnel systems in Vietnam btw). These networks don´t only provide tactical and strategical advantages but also are full of supplies, power stations, rest rooms, ect. The units of the Korean People´s Army are trained and supposed to fight independently. There is no cutting supplies. The mountains, caves and tunnels make air support less effective and there are traps everywhere. Also, the number of soldiers surpasses any coalition Trump could create.Depends if China will support them and we can cut off their suppliesOnly for the US. The US lost in Vietnam and they would lose in North Korea. The mountains, the tunnels, millions of soldiers. The first Korea war is a good example for that the destruction of the cities by heavy bombings will not win the war, particularly now as the North Koreans are prepared for the US warfare.
N Korea lacks the economy for a sustained war
this is just plain common military knowledge