Successfull merging of a subtangle and the main tangle: https://iota.stackexchange.com/q/1523/10 The case, that you have to reattach your whole subtangle occurs when you start your subtangle at a transaction that is not very likely to be confirmed and conficting with the main tangle.  In this example, the two transactions marked with Xs are conflicting and the network/coordinator chose the white transaction to be the valid one. Therefore the red transaction and all transaction referencing it, also your subtangle, are invalid and have to be reattached at another transaction in the tangle. That means the PoW has to be done again.  --- I created the tangles with the [yEd Graph Editor](https://www.yworks.com/products/yed). Source files can be found [here](https://gist.github.com/za-uz/8d7017c4a6928ec2edd88593be5df13f).