Last year the US
downed an Iranian drone near one of it's warships with technology that disabled the drone. I don't know the specifics of what the US ship did, nor the underlying technology. Imagine a country could prevent another country's war planes from taking off (or from continuing to fly), or could send a signal to change the course of another country's missiles that have been launched.
That's obviously impressive and potentially concerning. We do need to remember though that the advantages of quantum computers are limited to very specific areas, such as prime factorisation. In other areas they are no better than normal classical computers. Certainly the ability of QCs to break asymmetric cryptography could wreak havoc, but P-QC does offer solid defences, so - and I may be being naive - I think that the abilities of QCs are sometimes overstated, and critical systems can be protected, it's just a case of getting that protection implemented in time.
A drone needs to communicate with someone on the ground to receive instructions on how to operate. If this communications channel is not encrypted, anyone could send instructions to the drone to tell it what to do.
A QC could possibly crack whatever encryption is being used to communicate with the drone, then another computer could use the now found decryption key to communicate with the drone, and give it instructions to shut off its motor, or whatever.
So any military using QC in the battlefield would need to use QC, plus additional technology. I believe the additional technology is already widely available.