So his immediate reaction is to raise an army to defend the vacated land to his North? Not send an emissary to Castle Black to voice his objections? This came out of nowhere.
Then to align with the Bolton's who have shown themselves to be untrustworthy, it just makes no sense.
Voice concerns? Jon is the first Lord Commander ever, in the history of the NW, thousands of years to let Wildings into Westeros. The same Wildlings that just attacked the Wall and killed the majority of the NW in season 4. The same Wildlings that raided the North, pillaged, raped, and ate women and children recently (Ollys people were just one of many). It's too late to Voice concerns. This is war. It would be like if a military today just suddenly crossed the borders of another rivals sovereign nation. They don't send the ambassador to ask why.
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It's a declaration of war.
The people in the North don't think of Tormound or Ygritte when they think of Wildlings. They don't think of the pal Wun Wun when they think of Giants. They don't see free folk wanting a better life, they see the Thenss,
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soulless cannibals that don't reason, don't kneel, dont compromise. And now Jon Snow has joined them.
You say the Boltons have shown themselves to be untrustworthy? Fair. But they have also shown to be the best army in the north, currently, especially after they just wiped the floor against Stannis who had been considered one of the best military leaders in Westeros.