The Colonel will say: "¡Firmes!", and in English, "Attention!" or "Stand to attention". "Mande firmes" is telling him to give this order, so in English this would be "Call the men to attention" or simply "Call them to attention".

"firme
6. m. pl. Mil. Postura que consiste en estar parado con los tacones juntos, los pies en escuadra y los brazos rígidos pegados al cuerpo.
[...]
firmes: U. para ordenar que se adopte la postura de firmes."
http://buscon.rae.es/draeI/SrvltGUIBusUsual?TIPO_HTML=2&TIPO...

"call someone to attention
to demand that someone assume the formal military stance of attention.
The officer called the platoon to attention. The sergeant called us to attention."
http://idioms.thefreedictionary.com/call to attention

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Note added at 1 hr (2012-02-20 11:26:09 GMT)
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This phrase, "Mande firmes", is what the Spanish Minister of Defence says to the officer in command when reviewing the troops:

"Sólo ha pronunciado una frase: "Capitán, mande firmes". Por primera vez una mujer ha pasado revista a las tropas como ministra de Defensa."
http://blogs.periodistadigital.com/politica.php/2008/04/14/c...