Theoretically, will there be no more world wars nor regional wars if all countries are either communist / dictatorial or democratic in practice and ideology?

No, no and no.
Even if you are truly communist, (which, in its actual state is democratic by definition), with the state and money abolished, and the workers having direct ownership and control of the economy, there is no reason to think that regional groups will not develop different goals and aspirations from the others, and thus will have a reason to wage war against another group for either their resources or to force their will upon them, or have waged war against themselves to bring them “back in line”.
Democracies also have a well established history of waging wars against other democracies, for reasons from basic resource and territory acquisition, to minor and major differences in ideology, so there is no reason to believe that would change in any way, even if all countries were democratic.
Dictatorships are equally, probably more prone to bellicosity, considering they need an enemy at all times to keep the populace in line and unified under them.

War, violence, bloodshed. Greed and envy. In group/out group dynamics. Tribalism. All are at the very core of human nature, and that will never change.

^^ What he said.

@ragingloli Is correct. Enemies are the glue that holds super powers together. Without enemies a country will fracture, and will turn on itself.

Communism is a pipe dream. It cannot happen in reality for any significant length of time.

I think in theory, you could have some way of organising socially that eliminates any impetus for war and mass violence, or even individual violence.

On human nature, I am far more optimistic than @ragingloli.

My pessimism is more in ever reaching a more ideal society, because current systems that produce so much dysfunction are very entrenched, and the problem with human nature is more around bias toward the status quo and parochialism, which means there is a resistance to ever adopt anything different at all.

No.

Nope.

“Imagine no possessions.
It isn’t hard to do.
Nothing to kill, or die for.
And no religion too.”

@MrGrimm888 Possession is 9/10ths of the law. Is it that 9/10ths of the laws on the books are about possessions? Or that 9/10th of who owns something is the right of who currently owns the thing?

^In that context, that is saying that if there is a question of ownership of something, the person actually possessing the whatever, (in theory) gives the advantage in the dispute to the current owner.

At any rate, possessions, and the fact they are usually not plentiful enough for all, have constantly caused problems in human society.
Traits like jealousy, greed, and selfishness, are things humanity apparently cannot regulate.
For many, there is NEVER enough.
Often the disparity in wealth/resources, is deemed to lead to many problems like theft, violence, and war.

Countless millions of people, have died over some“thing.”
Unfortunately, humanity is not advanced enough, for people not to fight for more than they need. And therefore people have to fight, for what they possess.

Religion is self explanatory, as far as the way people cannot possibly coexist, with different religious beliefs.

Despite what some think, there are plenty of resources to support billions of people.
They are simply severely mismanaged.