Milites labore deficiunt.

Milites is nominative plural, so it is the subject of the sentence: the soldiers.

Its dictionary form is miles, militis = soldier.
This is a 3rd-declension noun.

In this sentence:

• milites = the soldiers
• it tells us who is doing the action

Because miles is a 3rd-declension noun, and its nominative plural form is militēs.

A learner may expect a plural like -i, but that is common for many 2nd-declension masculine nouns, not for this one.

So:

• singular: miles = soldier
• plural: milites = soldiers

Labore is ablative singular of labor, laboris, meaning work, toil, exertion, hardship.

Here it is an ablative of cause (or means, depending on how a textbook explains it). The idea is that the soldiers are failing or giving out because of exertion / from exhaustion.

So labore answers a question like:

• because of what?
• from what cause?

That is why Latin uses the ablative here rather than a preposition.

Latin often uses the ablative by itself where English would use a preposition such as:

• from
• because of
• with

So instead of saying something like from toil with a separate word for from, Latin can simply say labore.

This is very common in Latin. English usually needs extra words; Latin often does not.

Deficiunt is the 3rd person plural present active indicative of deficio, deficere, defeci, defectum.

Because it is 3rd person plural, it matches the plural subject milites.

So grammatically it means:

• they fail
• they give out
• they collapse
• they are giving way

Depending on context, it can also have the sense they are worn out or they are exhausted.

The ending -iunt tells you it is 3rd person plural.

Compare:

• deficit = he/she/it fails
• deficiunt = they fail

Since milites is plural, the verb must also be plural, and deficiunt fits that perfectly.

In Latin, the verb ending usually tells you the subject person and number, so a separate subject pronoun is often unnecessary.

So deficiunt already contains the idea they.
Latin can still add the noun milites to make clear who the they are.

That is very normal Latin style.

Yes, they are related.

Latin deficio literally has the idea of failing, giving way, breaking down, or falling short. English words such as defect, deficient, and deficiency come from the same root.

But in this sentence, deficiunt is best understood in context as something like:

• they give out
• they weaken
• they collapse from exertion
• they are exhausted

So the exact English wording depends on the translation already given, but the core idea is failing under strain.

Yes. Latin word order is much freer than English word order because the endings show the grammatical roles.

This sentence could appear in other orders, such as:

• Labore milites deficiunt
• Deficiunt milites labore

All of these could mean essentially the same thing.

The given order is straightforward:

• Milites = subject first
• labore = cause or means
• deficiunt = verb last

That verb-final position is very common in Latin prose.

They are:

• miles, militis = soldier
• labor, laboris = labor, toil, exertion
• deficio, deficere, defeci, defectum = fail, give way, break down

Learning dictionary forms is important because Latin words change shape a lot depending on case, number, tense, and person.

Grammatically, the ablative can express several related ideas, so labore could be described in different ways depending on the context and the textbook:

• by toil
• with toil
• from toil
• because of toil

But with deficiunt, the natural sense is usually from exhaustion or because of exertion. In other words, the soldiers are giving out as a result of toil.

So while the ablative label may vary, the meaning in context is what matters most.

Yes, it is the same word historically, and the meanings are very similar.

Latin labor can mean:

• work
• toil
• effort
• hardship
• fatigue

In this sentence, it especially suggests hard exertion or fatiguing effort, not just ordinary work.

A common classroom pronunciation would be roughly:

MEE-li-tays la-BOH-ray deh-FEE-kee-oont

More carefully in restored classical pronunciation:

• milites = MI-li-tes
• labore = la-BO-re
• deficiunt = deh-FI-kee-oont

The exact pronunciation depends on whether your class uses classical or ecclesiastical pronunciation, but the grammar and meaning stay the same.