Yes, it is permissible for the mujaahideen to break their fast in Ramadaan, so that they will have strength for jihad, even if they are in their own country, for fasting weakens their ability to fight and deal with the enemy.
This is one of the two opinions of Imam Ahmad and was the view favoured by Shaykh al-Islam Ibn Taymiyah and his two students Ibn Muflih and Ibn al-Qayyim, and other scholars. See al-Furoo’ by Ibn Muflih, 3/28
There is evidence narrated from the Prophet (peace and blessings of Allaah be upon him) that indicates that it is prescribed to break one’s fast because of jihad.
Muslim (1120) narrated that Abu Sa’eed al-Khudri (may Allaah be pleased with him) said: We traveled with the Messenger of Allaah (peace and blessings of Allaah be upon him) to Makkah – meaning at the Conquest of Makkah – and we were fasting. We stopped to camp and the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “You are approaching your enemy and breaking the fast will make you stronger.” This was a concession, and some of us continued to fast and some of us broke our fast. Then we stopped to camp again, and he said, “You are going to meet your enemy in the morning, and breaking the fast will make you stronger, so break your fast.” So we had no choice but to break our fast.
Abu Dawood (2365) narrated that one of the companions of the Prophet (peace and blessings of Allaah be upon him) said: I saw the Messenger of Allaah (S) telling the people on a journey in the year of the Conquest (of Makkah) to break their fast. He said, “Gain strength to face your enemy.”
Al-Haafiz said in al-Talkhees al-Habeer: this was classed as saheeh by al-Haakim and Ibn ‘Abd al-Barr
These two hadeeths indicate that the command to break the fast was not because of travelling, rather it was in order to gain strength for jihad.
It says in al-Muntaqa Sharh Muwatta’ al-Imam Maalik:
The words “Gain strength to face your enemy” indicate that this was the reason breaking the fast. If the reason was travelling then he would not have explained it as being in order to gain strength to face the enemy, rather he would have said it was because of travelling.
Al-Manaawi said in Fayd al-Qadeer:
He said, “You are going to meet your enemy in the morning.” According to another version, “You have come close to your enemy…” From the fact that he gave the reason as their approaching the enemy and their needing physical strength to meet their enemy, it may be understood that in this case the fast was broken because of jihad, not because of travelling. If the enemy attacked them in their own land [i.e., when they were not travelling] and they needed physical strength, then it would be permissible for them to break their fast, based on what is said, because that is more necessary than breaking one's fast just because of travelling.
Ibn al-Qayyim said in Zaad al-Ma’aad (2/53-54):
The Prophet (peace and blessings of Allaah be upon him) commanded them to break their fast when they got close to their enemy so that they would have the strength to fight them. If such a situation happened when they were not travelling, and breaking their fast would make them stronger to face their enemy, would they be allowed to break their fast? There are two opinions on this matter, the one which is based on the soundest evidence is that they are allowed to do that. This is the view favoured by Ibn Taymiyah, and this is the view stated in the fatwa he issued to the Muslim troops when they met the enemy on the outskirts of Damascus. Undoubtedly breaking the fast for that reason is more necessary than breaking it for travelling alone, rather the fact that the traveller is allowed not to fast alerts us to the fact that it is allowed in this situation (i.e., jihad), for it is more apt that it be allowed in this case because physical strength in that case only benefits the traveller himself, whereas in this case it benefits the mujahid and the Muslims, and because the hardship of jihad is greater than the hardship of travel, and because the interest served by the mujahid not fasting is greater than the interest served by the traveller not fasting. And Allaah says (interpretation of the meaning):
“And make ready against them all you can of power”
Breaking one’s fast when meeting the enemy is one of the greatest means of strength and power… and the Prophet (peace and blessings of Allaah be upon him) said, when they drew close to their enemy: “You are approaching your enemy and breaking the fast will make you stronger.” This was a concession, and some of us fasted and some of us broke our fast. Then we stopped to camp again, and he said, “You are going to meet your enemy in the morning, and breaking the fast will make you stronger, so break your fast.” So we had no choice but to break our fast.
So he gave the reason that they were approaching the enemy and they needed their physical strength to meet the enemy. This is a reason other than travel. Travel is a separate reason in and of itself, and he did not mention it or refer to it here. So in conclusion the principles and wisdom of the Lawgiver dictate that breaking one's fast for the purpose of jihad is more necessary than doing so simply because of travelling. So how about if the reason is explained and stated clearly in the text? The Prophet (peace and blessings of Allaah be upon him) insisted that they should break their fast for that reason, and this is indicated by the report narrated by ‘Eesa ibn Yoonus from Shu’bah from ‘Amr ibn Dinar who said: I heard Ibn ‘Umar say: The Messenger of Allaah (peace and blessings of Allaah be upon him) said to his companions on the day of the Conquest of Makkah: “This is a day of fighting, so break your fast.” He named fighting as the reason, and used the word “fa” (= so) when issuing the command to fight. From this wording each of them understood that breaking the fast was for the purpose of fighting.
And Allaah knows best.
The battle mentioned by Ibn al-Qayyim (may Allaah have mercy on him) took place between the Muslims and the Tatars in 702 AH, and the Muslims were the victors.
Ibn Katheer (may Allaah have mercy on him) said:
He – meaning Ibn Taymiyah (may Allaah have mercy on him) – issued a fatwa telling the people to break their fast for the duration of the fighting, and he himself also broke his fast. He used to go around among the soldiers and commanders, eating some food that he had in his hand, to show them that it was better for them to break their fast so that they would have more strength to fight. Then the people ate.
See al-Bidaayah wa’l-Nihaayah, 14/31.