der Legionär
legionary
A soldier who served in a legion of the ancient Roman Empire.
Die Disziplin und Ausbildung der römischen Legionäre war legendär.
The discipline and training of the Roman legionaries was legendary.

legionary
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legionary
A soldier who served in a legion of the ancient Roman Empire.
Die Disziplin und Ausbildung der römischen Legionäre war legendär.
The discipline and training of the Roman legionaries was legendary.

A 'Legionär' (legionary/legionnaire) is always a member of a 'Legion' (legion). Understanding the word 'die Legion' as a large military or figurative unit is the key to all meanings.
Jeder Legionär war Teil einer größeren Einheit, der Legion.
Every legionary was part of a larger unit, the legion.
The context reveals the meaning: historical texts about Rome mean the ancient soldier. Reports about the French military mean the Foreign Legion soldier. In the sports section of the newspaper, the foreign professional athlete is meant.
Im Geschichtsbuch las er über römische Legionäre.
In the history book, he read about Roman legionaries.
When soldiers of the Roman Empire are being discussed, 'Legionär' (legionary) is often used together with the adjective 'römisch' (Roman) or in a clear historical context involving Caesar, Augustus, and the Imperium Romanum.
Ein römischer Legionär trug eine schwere Rüstung und ein Kurzschwert.
A Roman legionary wore heavy armor and a short sword.
This meaning is specifically connected with the French 'Légion étrangère' (Foreign Legion). The term 'Fremdenlegion' (Foreign Legion) is often mentioned in the same sentence or text.
Er diente fünf Jahre als Legionär in der französischen Fremdenlegion.
He served for five years as a legionnaire in the French Foreign Legion.
The use in sports is a metaphor and belongs to sports jargon. A player who 'fights' for a foreign club is figuratively compared to a soldier in a foreign legion.
Der Verein hat sein Budget erhöht, um internationale Legionäre zu verpflichten.
The club increased its budget in order to sign international foreign players.
soldier
'Soldat' and 'Legionär' can both refer to a person in the military. 'Soldat' is the general word for someone who belongs to the armed forces. 'Legionär' is more specific: it can be a soldier in a legion of ancient Rome or a soldier in the French Foreign Legion. So every 'Legionär' in this sense is a 'Soldat', but not every 'Soldat' is a 'Legionär'. Learners may confuse the words because both refer to military people. In a normal modern context, people usually say 'Soldat'. 'Legionär' only fits when it is really about a legion, for example in history or the Foreign Legion.

Im Museum erklärt der Führer, wie ein römischer Legionär ausgerüstet war.
At the museum, the guide explains how a Roman legionary was equipped.
Der Soldat kommt nach mehreren Monaten Einsatz nach Hause.
The soldier comes home after several months on deployment.