is it legal to fight in war if you are drunk?

It is against the UCMJ to be drunk while on active duty. To wit:

"ARTICLE 134 - DISORDERLY CONDUCT, DRUNKENNESS
An enlisted member or officer who exhibits disorderly conduct, drunkenness or the influence of alcohol, or both while on active duty at an alleged place like an army camp, station, base, an aircraft or a ship, at an alleged time is in violation of the Article 134 and shall be punished as considered appropriate by the military court. "

And there you are. The official stand is...no you can't be drunk while on active duty.

But in practice, that's not always the case. For example some of my best passes while making carrier landings were made while totally hung over and bit buzzed. It's a long story, but the crux was that we ended up with an early morning recovery when it was originally planned to be a late afternoon trap aboard the aircraft carrier.

And I frequently drove through the streets of Saigon to the BOQ from Tan San Nhut airbase in Vietnam while three sheets to the wind (Navy talk for drunk). That was in 1969, during the height of the war.

My point, what is supposed to be is not always what it is...when it comes to being drunk in the military.

In Soviet Union, it was basically a requirement. If fought in Afghanistan 1982-84 for Soviet Union. We were given 50ml of vodka as a daily ration. That's about 1.6 oz. Not drunk, but we did drink. In 1983, I was given a 4 day pass in Uzbekistan, which was a muslim republic, but where we stayed, we were given 100ml of vodka daily. We were not allowed off the base, they said that we'd find no alcohol anyhow... I got to use a telephone to talk to my family, I got to eat really good and I slept a lot.
There were days that the mujaheddin really gave it to us. There were days we really gave it back. Sometimes after an operation, we'd also get the 100ml double ration.
After returning home, I drank a lot. Even when I made it to USA in 1992, I was still drinking. Since about 1995, I have not had one drink.

You'll fit right in! Seriously, who goes into war WITHOUT being drunk? I'd think you'd kinda have to be. That way when you die you won't feel a thing.

There were soldiers in World War Two who had to be pretty drunk to get out of the foxhole and charge the enemy.

In WW TWO it was quite common for the bomber crews to take off either drunk or really hung over. They had a 25%-50% chance of not coming back so the commanders would not fault them for partying each night like it was their last. Today they most likely would ground you for being drunk on duty. Only if you were the only one available and the mission was vital would they let you go drunk.