El enemigo cruzó la frontera.

El means the, so el enemigo means the enemy.

Spanish usually uses the definite article much like English does here. If you said un enemigo, that would mean an enemy or one enemy, which is less specific.

• El enemigo cruzó la frontera. = The enemy crossed the border.
• Un enemigo cruzó la frontera. = An enemy crossed the border.

Enemigo is the masculine singular form of enemy. Spanish nouns and adjectives often change form depending on gender.

• el enemigo = the male enemy / the enemy (masculine)
• la enemiga = the female enemy

In some contexts, the masculine form can also be used in a general sense, especially if the enemy is not identified as female.

Cruzó is the preterite tense, third person singular, of cruzar (to cross).

It means that the action was completed in the past:

• cruzó = he crossed / she crossed / it crossed

In this sentence, it tells you that the crossing happened as a finished event.

The accent mark shows both pronunciation and meaning.

• cruzo = I cross (present)
• cruzó = he/she/it crossed (preterite)

So the accent helps distinguish two different verb forms. It also shows that the stress falls on the last syllable: cru-ZÓ.

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

• cruzó already tells you it is he/she/it
• So El enemigo cruzó la frontera is completely natural

You could say Él cruzó la frontera, but only if you wanted to emphasize he. In this sentence, el enemigo already names the subject, so él is unnecessary.

Frontera is a feminine noun, so it takes the feminine article la.

• la frontera = the border

Many Spanish nouns ending in -a are feminine, and frontera follows that common pattern.

The personal a is used before a direct object that is a person (or sometimes a pet or personified being).

Here, la frontera is a thing, not a person, so there is no personal a.

• Veo al enemigo. = I see the enemy.
  (Here a appears because enemigo is a person.)
• Cruzó la frontera. = He/She crossed the border.
  (No a, because frontera is not a person.)

In standard Spain Spanish, it is pronounced approximately:

el e-ne-MI-go kru-THO la fron-TE-ra

A more phonetic version is:

[el e.neˈmi.ɣo kɾuˈθo la fɾonˈte.ɾa]

A few useful pronunciation points:

• z in cruzó is pronounced like th in think in most of Spain
• the r in frontera is a tapped Spanish r
• stress falls on:
  • enemigo → mi
  • cruzó → last syllable
  • frontera → te

Yes. Atravesó la frontera is also correct and natural.

Both cruzó and atravesó can mean crossed, but there is a slight nuance:

• cruzar = to cross
• atravesar = to go across / pass through / cross from one side to another

In many contexts, they are interchangeable. Cruzó la frontera is very direct and common.

No, Spanish word order is more flexible than English, although subject + verb + object is the most neutral order.

The sentence as given:

• El enemigo cruzó la frontera.

is the standard, neutral way to say it.

But Spanish can change the order for emphasis:

• La frontera la cruzó el enemigo.
  This gives special emphasis to the border or creates a more literary/dramatic tone.

For a learner, the original word order is the best default.

Yes, sometimes el enemigo can refer to the enemy as a collective force, much like in English.

So depending on context, it might mean:

• one enemy
• the enemy side / opposing force

However, because the verb is cruzó (singular), the grammar treats el enemigo as a singular subject. If you wanted to make the plural explicit, you would say:

• Los enemigos cruzaron la frontera. = The enemies crossed the border.

The infinitive is cruzar, which means to cross.

Some useful forms are:

• cruzar = to cross
• cruzo = I cross
• cruza = he/she crosses
• cruzó = he/she crossed
• cruzaron = they crossed

This is a regular -ar verb in the preterite:

• yo crucé
• tú cruzaste
• él/ella cruzó
• nosotros cruzamos
• vosotros cruzasteis
• ellos/ellas cruzaron

Yes, it is fully natural and correct.

It is also a slightly formal or narrative-type sentence, because of the vocabulary:

• enemigo = enemy
• frontera = border

You might see it in history, news, military contexts, or storytelling. Grammatically, though, it is simple and very standard Spanish.