I recently experienced the same thing wish Choshu, and unlike all people above, I strongly recommend to not to back-stab your Imperial allies. Once you conquered enough provinces they will be your allies and help you on your cause so greatly.
Now my empire looks like a patchwork: It literally consists of 6 isolated parts, plus an island. The gaps are filled with my allies. We march together against Shogunate.
What you should be doing is be an ally with them, and grant each other military accesses. When your army get exhausted, they will continue marching from your land.
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