It is permissible to join Zuhr and ‘Asr at the time of the earlier or later prayer in cases such as this, because joining prayers is a concession that is granted. So whenever one needs to join them, one may join them. Hence it is proven in al-Saheeh from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) joined prayers in Madeenah at times other than times of fear or rain. It was said to Ibn ‘Abbaas: Why did he do that? He said: So that his ummah would not be faced with hardship., i.e., that they should not suffer hardship because of offering each prayer on time. End quote from Fataawa Noor ‘ala al-Darb by Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).
You have to advise those who are in charge of these manoeuvres to choose times for them that do not conflict with the prayer, so that the soldiers can join Zuhr and ‘Asr at the time of either the earlier or later prayer.
May Allaah be pleased with ‘Umar ibn al-Khattaab, who wrote to his governors in the regions: The most important of your affairs in my view is the prayer. Whoever prays regularly has protected his religious commitment, but whoever is careless about it will be more careless about other things. Narrated by Maalik in al-Muwatta’ (6).
And Allaah knows best.