Yemen vs. Oman

as of 2020-2021, military and security forces operating in Yemen consists of a patchwork of government, non-state, and foreign-backed regular, semi-regular/militia, and paramilitary forces, often with informal command organizations and conflicting, fluid, or overlapping agendas, loyalties, and relationships; in addition, the Huthis maintain opposition military and security forces

Republic of Yemen Government (ROYG) forces; Ministry of Defense: Yemeni National Army, Air Force and Air Defense, Navy and Coastal Defense Forces, Border Guard, Strategic Reserve Forces (includes Special Forces and Presidential Protection Brigades, which are under the Ministry of Defense but responsible to the president), Popular Committee Forces (government-backed tribal militia); Ministry of Interior: Special Security Forces (paramilitary; formerly known as Central Security Forces), Political Security Organization (state security), National Security Bureau (intelligence), Counterterrorism Unit

Saudi-backed forces: paramilitary/militia border security brigades based largely on tribal or regional affiliation (deployed along the Saudi-Yemen border, especially the areas bordering the governorates of Saada and Al-Jawf)

United Arab Emirates-backed forces include tribal and regionally-based militia and paramilitary forces (concentrated in the southern governates):  Southern Transitional Council (STC) forces, including the Security Belt Forces, the Shabwani and Hadrami "Elit-e" Forces, the Support and Backup Forces (aka Logistics and Support Forces), Facilities Protection Forces, and Anti-Terrorism Forces; Republican Forces; Joint Forces; note - under the 2019 Riyadh Agreement, the STC forces are to be incorporated into Yemen's Ministries of Defense and Interior under the authority of the HADI government

Huthi: ground, air/air defense, coastal defense, presidential protection, special operations, missile, and tribal militia forces

the Maritime Administration of the US Department of Transportation has issued Maritime Advisory 2020-017 (Red Sea, Gulf of Aden, Arabian Sea, and Indian Ocean-Violence Due to Regional Conflict and Piracy) effective 14 December 2020, which states in part the "Conflict in Yemen continues to pose potential risk to US flagged commercial vessels transiting the southern Red Sea, Bab al Mandeb Strait, and Gulf of Aden. Threats may come from a variety of different sources including, but not limited to, missiles, rockets, projectiles, mines, small arms, unmanned aerial vehicles, unmanned surface vessels, or waterborne improvised explosive devices. These threat vectors continue to pose a direct or collateral risk to US flagged commercial vessels operating in the region. Additionally, piracy poses a threat in the Gulf of Aden, Western Arabian Sea, and Western Indian Ocean."

the Maritime Administration of the US Department of Transportation has issued a Maritime Advisory (2021-003A Persian Gulf, Strait of Hormuz, Gulf of Oman, Arabian Sea, Red Sea-Threats to US and International Shipping from Iran) effective 26 February 2021, which states in part that "heightened military activities and increased political tensions in this region continue to present risk to commercial shipping...there is a continued possibility that Iran and/or its regional proxies could take actions against US and partner interests in the region"; Coalition Task Force (CTF) Sentinel has been established to provide escorts for commercial shipping transiting the Persian Gulf, Strait of Hormuz, and Gulf of Oman

information limited and widely varied; ROYG: up to 300,000 estimated military, paramilitary, and militia forces; UAE- and Saudi-backed forces: 150-200,000 estimated trained fighters; Huthis: up to 200,000 estimated fighters (2021)