The transfer of drones to Ukraine offers the greatest benefit and lowest escalatory risk among 11 choices for U.S. and allied action, including humanitarian assistance, military equipment assistance, and covert/overt military action in Ukraine, according to a new Atlantic Council report by U.S. Air Force Lt. Col. Tyson Wetzel, a senior Air Force fellow at the council’s Scowcroft Center for Strategy and Security, and Barry Pavel, the director of the Scowcroft Center.

The study compiled the results of a survey of 37 “national security experts, including a former ambassador to Russia and top NATO official, former senior officials at the U.S. National Security Council and Defense Department, retired and active-duty military personnel, and experts across the Atlantic Council.”

“The experts we surveyed assessed the transfer of UAVs to Ukraine as the most effective option relative to the risks of escalation,” per the study, which said that continued provision of Turkish TB2 Bayraktar drones would be especially beneficial.

“The consensus of respondent comments indicated that the TB2 UAV is already having decisive effects on the battlefield, and that rapid deployment of many more of these systems would help the Ukrainian military halt Russian momentum,” the study said. “The majority of our respondents also did not believe that the transfer of these UAVs to Ukraine would be particularly escalatory.”

Another option in the survey was the transfer of U.S. Patriot missile systems by Raytheon Technologies‘ [RTX] to Ukraine. Survey respondents judged such a transfer would likely have great benefit in providing missile and air defense for Ukraine but that the option would carry a high risk of escalation and would impose a significant training burden on U.S. forces.

This week, Air Force Gen. Tod Wolters, the head of U.S. European Command, ordered the movement of two Patriot batteries from the 10th Army Air and Missile Defense Command at Rhein Ordnance Barracks in Germany to Poland. Those batteries are in place in Poland, the Pentagon said on March 9.

While providing combat aircraft to eastern flank NATO allies and establishing a U.S./NATO “no fly zone” over Ukraine are other options, the Atlantic Council and U.S. officials have judged those choices as having high escalation potential.

“We understand Poland recently made various proposals to transfer its MiG-29 aircraft from Poland to assist Ukraine,” Wolters said in a March 10 EUCOM statement. “We believe the most effective way to support the Ukrainian military in their fight against Russia is to provide increased amounts of anti-tank weapons and air defense systems, which is on-going with the international community. The Ukrainians are making excellent use of these weapons now. Although Russian air capabilities are significant, their effectiveness remains limited due to Ukrainian strategic, operational, and tactical ground-based air defense systems…SAMs and MANPADS. ”

“USEUCOM assesses the military usefulness of additional fixed wing air to Ukraine will be high-risk and low gain,” Wolters said.  “The transfer of MiG-29 aircraft will not appreciably increase the effectiveness of the Ukrainian Air Force. The Ukrainian Air Force currently possesses numerous mission capable aircraft that are flying daily. Adding aircraft to the Ukrainian inventory is unlikely to change the effectiveness of the Ukrainian Air Force relative to Russian capabilities. Therefore, we assess that the overall gain is low. Lastly, the intelligence community assesses the transfer of MiG-29s to Ukraine may be mistaken as escalatory and could result in Russian escalation with NATO…producing a high risk scenario.”

After the transfer of UAVs to Ukraine, the Atlantic Council survey “identified the transfer of electronic-warfare equipment, counter-fire systems, and the CIWS [Close-In Weapons System] and Iron Dome air-defense systems as the next most advantageous options.”

Raytheon builds CIWS, while Rafael Advanced Defense Systems and Israel Aerospace Industries are the contractors for Iron Dome.

“Any actions that involved U.S. or NATO personnel deploying to conduct operations inside Ukraine, even humanitarian operations, were rated as relatively more escalatory than militarily effective, with the riskiest being SOF [Special Operations Forces] operations,” the Atlantic Council study said.

The council offered anonymity to survey participants, but the ones named in the report include the co-authors; Alexander “Sandy” Vershbow, a former deputy secretary general of NATO and former U.S. ambassador to Russia; Ian Brzezinski, a former U.S. deputy assistant secretary of defense for Europe and NATO policy; Rasmus Hindrén, the head of international relations at the European Center of Excellence for Countering Hybrid Threats and a former senior official in Finland’s Ministry of Defense; Hans Binnendijk, a former senior director for defense policy and arms control on the U.S. National Security Council; and Ronald Marks III, a professor at George Mason University and a former clandestine service officer and special assistant to the assistant director of central intelligence for military affairs.