Lines 1-9 Summary

Get out the microscope, because we’re going through this poem line-by-line.

If I had not met the red-haired boy whose father

The poems starts out, interestingly enough, with a conditional ("if") clause. We expect it to have a second part to it. For example, "If I had not gone to that restaurant, I wouldn't have gotten food poisoning."
We use "if" clauses to create expectancy or suspense, which can be positive or negative. (The expectancy you feel if you leave cookies out for Santa Claus is positive, because you think he'll come fill your stockings with presents. But the expectancy you feel if your boyfriend doesn't text you back is negative, because it means he's probably going to dump you. Bummer.)
So, we want to know what the result of the "if" is. We're thinking the red-haired boy and his dad might be important, or at least relevant to our speaker, because they're mentioned in the first line.

had broken a leg parachuting into Provence
to join the resistance in the final stage of the war

Provence is in France, and the war our speaker is talking about here is World War II (extra, extra: Shmoop all about it).
So the red-haired kid's dad broke his leg parachuting into France, joining the Allied Forces against Germany in WWII. For more specific deets, check out Shmoop's World War II Timeline, especially April 1940: "Germany Pummels France."
We still haven't gotten the second part of our "if" clause yet. In fact, things are getting more complex. We've been thrown even more information. It's no longer a two-part (if I leave the cookies… then Santa leaves presents) clause. It's not just, "if I hadn't met the boy." It's, "If I hadn't met the boy whose dad got hurt in the war…" and so on. The "ifs" are starting to accumulate into a big snowball that's threatening to run us over before we ever find out the resolution. Help!
Luckily, we do start getting a sense of the time period our speaker is from. If he's from the generation of the red-haired boy, he's probably somewhere between 50 and 65, judging from the present (this poem came out in 1996) to WWII time relationship. MATH!

and so had been killed there as the Germans were moving north

In this line, Merwin is referring to the father with the broken leg. When he parachuted in, he hurt his leg, so he was probably an easy target for the German soldiers.
So instead of resolving the "if" clause, Merwin seems to be layering more and more onto it. We're going to have to keep tracing things back so we can keep track of what he's talking about.
So the red-haired boy's dead dad is somehow significant to the speaker (even though he never knew him). Huh. Let's keep going to see if this knot unravels…

out of Italy and if the friend who was with him
as he was dying had not had an elder brother

The build-up continues. Now we have a second "if." Sheesh!
The fact that so far there's no punctuation adds to the overwhelming feeling. We're getting all of this loosely connected information all at once, which has a bombarding effect. It's almost as if Merwin is playing that game where you go around a circle and everybody adds on to the sentence until it gets out of control and nobody can remember how it started. Hopefully we'll see how all of this connects to our speaker, you know, before the poem ends.
So the boy's father who died had a friend in the army, and that friend had an older brother. Everybody writing this down?

who also died young quite differently in peacetime
leaving two children one of them with bad health

Whoa. We are getting further and further away from our speaker. At this point, there are four degrees of separation. We're just dying to know how they connect to the speaker.
To be clear (as clear as we can be): the red-haired boy's dad's friend had an older brother who died, not in the war, but in peacetime, and also while he was still pretty young.
This older brother (who was not in the war) left two kids behind, one of whom was sickly. Still with us? Let's keep moving…

who had been kept out of school for a whole year by an illness

The sickly son of the red-haired boy's dad's friend's dead older brother (whew!) missed a year of school because his sickness was so bad.
The degrees of separation from the speaker at this point, and the amount of "ifs," is getting overwhelming!
We're still left wondering how to resolve the puzzle—if all of these things hadn't happened, then… what? And we're still wondering how they relate to our speaker. So far, we know nothing about our speaker, except in very distant connection to all of these mini-stories.