To wage a war in space you need 3 important resources
Thus sources of those resources can be strategical points.
A star system
A star system with Dyson swarm will supply with energy fairly well, and contains matter for millions of years if you rely on the star energy output. Million years mostly because one can mine the star itself and it is usually deep gravity well compared to the energies it produces and energies needed to lift the matter from the star.
Planets are obvious sources of different matter, especially Gas giants, as an example
So, in fact, any star system can and will be a strategical location just by itself. But stars have different energy output, and in that regard choosing stars with higher energy output might be preferable.
A supernova and supernova remnant
Less obvious choice, but in fact, it might be a very valuable resource.
It provides with energy and matter, and if one is capable of harvesting it(which needs a good technology, but still seems to be in realm of known to us physics)
The energy of a supernova blast is about 1044J (up to 1046J) which is equivalent to the energy produced by 8300992 stars like our Sun in 1000 years time.
A lot of the energy is stored in the kinetic energy of the star matter which is heading outwards.
That can be used in multiple ways, including launching your fleet to all stars of your enemies which is the most primitive thing you can do with it because the energy has way richer uses.
Not only ready to blast supernova is valuable, its remnant a black hole or neutron stars are also very valuable objects.
They are valuable at least for 2 reasons - energy, and matter. You can use their gravitational potential to convert available to you matter to the energy (both for BS and NS), or you can use NS as a neutron source for transmutation of the elements you have into elements you would like to have.
some thoughts about supernova as a useful resource in the answer My star will explode as a supernova. What can I do in order to ensure that my planet survives that?
A supermassive black hole
Mostly the same as with usual black holes or neutron stars - but at way much greater scale. Way much greater scale.
Basically, if you have the technology, which is still in the realm of known physics and known materials, by securing the SMBH you secure the galaxy.
You can throw stars into the SMBH to generate energy and energy extracted just from a single star like our sun thrown in SMBH will produce about 894 foe (1 foe = 1044 J)
A bit more why the SMBH are useful as energy producing places in the answer Why would a civilization choose to inhabit a single enormous vessel instead of maintaining interstellar colonies?
It is important to understand that scale of action and technologies aren't that heavily connected with each other, and it is in that way since we invented computers.
I mean, if you can operate on the scale of a single star system or even a planet - only physics stops you from scaling the technology to use it on 1000 star systems, or million star systems, or the galaxy.
The physics in terms of availability of matter for your devices and energy for them to work and energy needed to launch them to the target.
Defense is more or less the same it is a matter of how much you have both resources and energy.
Energy utilization efficiency of the technology, materials demands for the technology it affects the capabilities of those who use them, but in a lesser way that ability to get the access to an energy source which boosts you capabilities 6-10-20 order of magnitude compared to your enemy.
The gap between technologies of both parties might be significant in terms of how advanced it is, but the difference can be compensated by energies available to those parties. And to wage galaxy wars the technologies have to be above a certain level.
At least is seems that way at our current level of understanding of the physics, significant break trough may change equilibrium of involved parties
And thus megastructures like matryoshka brains might be strategical points too as they generate science for your enemy. And it is in your best interests to have them and prevent your enemy to have them.
The technology used in the answer is sufficient to wage wars at stars/galaxy level(K1, K2 civilizations).
Have read the answers, and it seems many do not understand which practical implications FTL communication have, by saying the ships will be older and technologically less advanced than enemies ships.
You have to understand the scale of such thing as attacking a star system. A system where you enemy can control everything, where it sees the attack is coming, it sees its size, where he has time to prepare for counterstrike using the whole resources of the star system.
Your attack can be successful if none of that matters when all the preparations of the defending side are totally useless. When he has no chances to defend himself using all resources he has in the system when they are against an overwhelming force heading to their system.
No matter in which technology do you believe, what you think is possible or not, but taking manufacturing capabilities in the case is the must for the attacker. You live in the time when all Intel facilities technologies could be sent to you with one mouse click, same can be done for the attacker fleet, they can improve themselves on the way to the target according to the technologies available to those who sent them.
With some technologies, it can be easier with other more of a hassle but it definitely possible.
I recommend Isaac Arthur Megastructures playlist, all.
To grasp the scale of what can be possible I highly recommend to watch his The Kardashev Scale - it is not about technology but about of scale of things how big they might be and some practical implications of the scale.