Very astute observation. Ukraine should obviously continue just fighting defensively because it worked once at the Seige of Malta (a tiny island). In fact I think they teach the Seige of Malta tactics at West Point as the bestest way for countries to succeed in repelling invaders, especially in waiting until they’re attacked by invaders over and over.
You asked a question. I answered. I would also point to the
Siege of Rome of AD 537 as another example. The Siege of Vienna as well.
In the the case of the Siege of Rome, the besieged held out long enough that the aggressors exhausted themselves trying to take the city. Once that happened, the besieged sent forces behind the enemy encamped around the city and threatened their supply lines, causing the aggressors to retreat in disarray.
If this is the type of strategy that Ukraine is employing, then more power to them.