1) A gap of 1 meter is acceptable, and it does not break the continuity of the Jama’a prayer. So yes pray with them Jama’a it’s highly recommended.
The Imams have told us that a gap of “big step” is ok, and won’t break the Jama’a prayer. Some scholars say this “big step” is 1.2 meters and some say 1.3 meters. As long as you are under 1.2 m your Jama’a prayer is valid Insha’Allah
2) We have a hadith that states it’s recommended not to have any gaps in Jama’a prayer, and to stand shoulder to shoulder. When Muslims stand should to shoulder, they feel more brotherhood, and that reduces the influence of Satan on them. But it’s ok to have a gap for a health concern, such as COVID-19.
The gap doesn’t have anything to so with war. In order to offer a proper Jama’a, there should not be a big gap (such as 1.2 meters). This doesn’t have anything to do with war.