The phrase "close alliance" means a strong and near relationship between two or more individuals, groups, or countries. They work together closely and support each other strongly. Full definition
The few who have been successful in obtaining this certification are a very close alliance with many things in common. (acfchefs.org)
Boarding schools have long been praised for offering students small class sizes, close alliances between students and teachers, and rigorous academics. (thoughtco.com)
And the Rhema church — again, to cite but one example — works in close alliance with several Sowetan evangelical ministries. (firstthings.com)