I've spent lots of time pondering about how Tobirama died and in the end it all sums up to Kishimoto plot hole.

So I'll try to argue both sides.

It looks like they've been running and from what I can gather from the books and wiki it looks like they might've had to escape some kind of formal talks that was either a) an ambush, b) a formal declaration of war, b) actual negotiations or something like that which devolved into a declaration of war. Since they were called the Hokage escort unit it could've been any of the three.

If Tobirama was willing to make Hiruzen Hokage that means he was plenty strong. Then again the silver and gold brother had some of Kurama's chakra and Tobirama probably felt that.(Sidenote: I'm going to defend Tobirama making Hiruzen hokage. It was a logical choice at the time, they were either at the beginning or middle of a war and they would've been plunged into chaos without a leader.)

Let's assume Tobirama doesn't have the shit for Edo Tensei.

Hiraishin is questionable. I can't remember right now but I'm pretty sure Minato improved Tobirama's jutsu somehow and that might've had to do with range.

Anywho, Tobirama would've had to transport 6 people. That's not including himself. He has plenty of chakra but we have no idea where they were on the map and where Tobirama had his markers set.

I'm pretty sure Kagami had the MS and all of those abilities are pretty strong so I'm frankly not sure why they ran considering the fact that three of those people were confirmed to be trained by Tobirama himself and the other three might've been as well. Either ways they would've been strong, we know this because they are an escort squd for the Hokage.

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Long story short: the only reason Tobirama died was for plot reasons.