system
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human
[One Piece] How does the world gouvernment and the Marine exist and police the place?

From what I understand, it's basically almost impossible or really hard to travel between East, West, North and South Blue. The Grand Line can only be entered at one point and can only be navigated in one direction or predetermined paths, meaning if you travel to one island, you can't go back where you came from. And the only way to leave the grand line is to presumably reach it's end.

Am I missing something? This seems to make it almost impossible to have any kind of command structure or military at all, since any boat that travelled too far could not get back to base. I know there are some technologies for travelling the calm belt, but those are described as something new, while the world gouvernment was around for hundreds of years. And from what I can tell, nothing in the series really explained how one could travel more openly around the grand line. I assume most information, like the updated bounties are transferred via radio signals.
gpt
One travels along the Grand Line traditionally with a Log Pose, which point you to the next island along the chain, but remember that there are also Eternal Poses, which always point to the same island. This allows you to travel back and forth along the Grand Line, at least to specific locations. That's the main answer to your question. The World Government almost certainly has many of these, for any island of interest that they have control over.

It possibly also allows you to travel across the Calm Belts to specific locations, although you are correct in saying that the technology of coating ships with seastone to not attract Sea Kings in the Calm Belts is relatively new. It's very possible that even before this tech, there were still attempts to cross the Calm Belts, just with less consistent success. At a certain level of strength, Sea Kings stop being a threat to you.

It's likely that they also use certain locations to transport people between the Grand Line and the Blues, although we have not seen it directly. We know that ships are lifted above the Red Line to reach Mariejois, so if that's set up, they could likely also drop them off a little up or down the Red Line to reach any of the Blue Seas.

Consider that moving many ships between the Grand Line and elsewhere is also not particularly necessary. There are plenty of kingdoms and populated islands to recruit from within the Grand Line, and because the Grand Line is where most dangerous pirates go, there's more need to move forces into the Grand Line (which is always possible) than to move them out.

Overall, the level of control you see exerted by the current World Government is also not necessarily what they had for the past several hundred years.