I have a world where the different countries feature various types of organized groups, some are criminal groups (mafia type organizations) and others are non-criminal but they are both armed and willing to fiercely defend themselves. This works quite well for in-fighting within a country, but what if one of these groups decided that a rival group in another country is a threat, has something they want, or they just want to kick their butt as a show of strength? How would a group like that have it's members travel to the rival's country, transport any weapons, vehicles etc to conduct said attack? Would they take their things with them from their "turf" or organize equivalents to be available near the rival's location to save on the transport logistics? If a large number of members needs to travel, would there need to be ways to not make this suspicious to authorities that something big is about to go down?
I would imagine that a better option would be to simply hire someone local to complete this task, so happy for a reality check here, but I would prefer it to be the group vs group scenario just because of the increased excitement and stakes of this kind of battle, over a simple sniper assassination of the rival boss. Also, neither side would have access to any sort of aircraft or military-like vehicles as a part of it's arsenal: just firearms, explosives and everyday vehicles.
The world is essentially very similar to modern day today in every way (minus COVID :), so the usual travel related security checks would be in place. I am also assuming that the two countries in question are at peace i.e. not active war zones. Happy to provide any further details on that, please just ask.