Introduction
How is the Military in Saudi Arabia and how does it work to maintain Sucity and fight terrorism? In terms of Military and security forces, we have the the Saudi Arabian Armed Forces (SAAF) are divided into two ministries:

Ministry of Defense: Royal Saudi Land Forces, Royal Saudi Naval Forces (includes marines, special forces, naval aviation), Royal Saudi Air Force, Royal Saudi Air Defense Forces, Royal Saudi Strategic Missiles Force; Ministry of the National Guard: Saudi Arabian National Guard (SANG)

Other security forces include: 

Ministry of Interior: Special Forces for Security and Protection; Special Forces for Environmental Security; Special Forces for Roads Security; General Directorate of Border Guard

State Security Presidency (SSP): General Directorate of Investigation (Mabahith), Special Security Forces, Special Emergency Forces (2024).
In Saudi Arabia, the Saudi Arabian Armed Forces (SAAF) are divided into the regular forces under the Ministry of Defense and the Saudi Arabian National Guard (SANG); the regular forces are responsible for territorial defense, although they can be called for domestic security duties if needed; they include land, naval, air, air defense, and strategic missile forces

the SANG is responsible for both internal security and external defense; its duties include protecting the royal family, guarding against military coups, defending strategic facilities and resources, and providing security for the cities of Mecca and Medina; the SANG is primarily comprised of tribal elements loyal to the Saud family and is comprised of brigades of light infantry, mechanized or motorized infantry, and security forces; it is supplemented by combat helicopter units and tribal levies/militias known as Fowj

Saudi Arabia has close security ties with the US; the SAAF conducts bilateral exercises with the US military and hosts US forces; the US has participated in a cooperative program to equip and train the SANG since 1973; much of the equipment for both the regular forces and the SANG has been acquired from the US; Saudi Arabia also has defense relationships with China, France, India, the UK, and fellow Gulf Cooperation Council members

in 2015, Saudi Arabia led a military intervention into Yemen by a coalition of Arab states in support of the Republic of Yemen Government against the separatist Houthis; Saudi forces from both the Ministry of Defense and the SANG participated in combat operations in Yemen; Saudi Arabia also raised and equipped paramilitary/militia security forces in Yemen--based largely on tribal or regional affiliation--to deploy along the Saudi-Yemen border (2024)
Security and military information of Saudi Arabia
Military and security service personnel strengths the Saudi Armed Forces have about 250,000 total active troops; approximately 125,000 under the Ministry of Defense (75,000 Land Forces; 15,000 Naval Forces, including about 3,000 marines; 35,000 Air Force/Air Defense/Strategic Missile Forces) and up to 125,000 in the Saudi Arabia National Guard (SANG) (2023)
Military equipment inventories and acquisitions the inventory of the Saudi military forces, including the SANG, includes a mix of mostly modern weapons and equipment from Europe and the US; the US has been the largest supplier; major European suppliers have included France, Spain, and the UK; Saudi Arabia is one of the world's largest importers of arms (2024)
Military service age and obligation men (17-40) and women (21-40) may volunteer for military service; no conscription (2023). note: in 2021, women were allowed to serve in the Army, Air Defense, Navy, Strategic Missile Force, medical services, and internal security forces up to the rank of non-commissioned officer
Military deployments continues to maintain a military presence in Yemen; has also established and supports several local militias, including the National Shield Forces in Aden and the Amajid Brigade in Abyan (2023).
Military expenditures
Military Expenditures 2023 7% of GDP (2023 est.)
Military Expenditures 2022 6% of GDP (2022 est.)
Military Expenditures 2021 6% of GDP (2021 est.)
Military Expenditures 2020 8% of GDP (2020 est.)
Military Expenditures 2019 8.8% of GDP (2019 est.)
Terrorism
In Saudi Arabia, the different Terrorist group(s) inlcude: Islamic State of Iraq and ash-Sham (ISIS); al-Qa’ida; al-Qa’ida in the Arabian Peninsula (AQAP)
All Important Facts about Saudi Arabia

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Saudi Arabia is found in Western Asia