Nations participating in a proxy war against another relation improves everyones relations toward them.
For example. Start as France. CHeck your relations with N. Veitnam BEFORE you even unpause it.
Now wait for Other nations to engage North veitnams troops. (keep yours back to make it easier, but other nations won't send any troops if you don't engage at least once)
Notice the first time one of your allies engages A north vietnamese troop it improves your relations with north vietnam. Eventually maxing out the diplomatic and Civilian relations of every one in the alliance toward North vietnam once enough allies join.

After 2 allied nations engage N. Vietnam. Frances relations toward vietnam = satisfied.
After 4 your Delighted with them..

I'm not sure your allies joining in a proxy war should improve your relations toward the country your at war with.
If it was a defensive situation it would be even worse.
Think country A attacks country B.
Country B's allies all join in to defend against the agression.
All of a sudden the whole world loves country A.
Add into this the fact that a country joining into a proxy war gives you C.B. against them.
So now country A attacks country B.
Country C is allied with country B. So shows up to help.
The world gains C.B. toward country C while everyone in the world loses C.B against country A.
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Si vis pacem, para bellum
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