“They cannot maintain themselves as they cannot feel a component before it fails.”
Internal self diagnostics. Already used on the Leopard 2 MBT to find faulty parts easily without disassembling the tank. Just plug in the laptop and look what is wrong.
“To defeat them you remove their power sorece”
You would have to get close enough to remove the power source. especially problematic if they have an internal powersource, like a hydrogen fuel cell, miniature nuclear reactor, zero point energy generator, etc.
“or introduce water”
To get water into their system, same problem, getting close enough. Also assumes their cases are not waterproof. Furthermore, their entire circuitry could be optical and and their actual power source isolated, and therefore impervious to water.
“Since they do not reproduce you need not destroy them all at once.”
They could reproduce by taking over/building a factory start producing parts and assembling offspring. Alternatively there is the option of nanotechnology “growing” their offspring out of rawmaterials.
“They are very delicate and vulnerable.”
You can drop cellphones from several metres high and they will only have a scratch.
Modern battletanks, fighter aircraft etc are full of electronics and computers and they are sturdy enough to withstand the stresses of a battlefield.