Main factors in why European countries buy defence equipment from abroad

Several long-term structural factors and short-term situational circumstances contribute to this imbalance in European defence procurement.

1. Politics: most defence acquisition decisions have a political rationale. Armament procurement is often used to reinforce a strategic and/or security partnership. This has traditionally been a strong argument for many Europeans to buy US equipment.
2. "FMS" effect: Foreign Military Sales (FMS) is a security assistance programme of the US government to facilitate the purchase of US defence equipment to foreign governments. European customers often use the FMS, as it offers several benefits, including a ‘package deal’ that entails seemingly reduced overall costs; training, maintenance, and logistical support; a streamlined, one-stop-shop procurement process that reduces procurement timelines and administrative burdens; the ability to derogate from EU procurement rules; and the possibility of financing and related security assistance programmes. These benefits can often tip the scales in favour of US suppliers.
3. Availability: following Russia’s invasion of Ukraine, short-term availability of purchased equipment has become a key concern for European governments and militaries. As a result of larger domestic market size (US) and/or higher defence readiness levels (South Korea), some non-European companies have been able to maintain greater production capacity and thus to deliver (or promise) larger volumes of equipment and at greater speed than European companies could after decades of downsizing and underinvestment. Countries such as the US had also maintained significant stocks and reserves (mothballed) of equipment that could be made available to partners and/or Ukraine relatively quickly. 
4. Legacy effects: past acquisitions can also shape future procurement decisions. Militaries may want to replenish and/or increase their stocks of existing systems, equipment, and ammunition. They may also prefer new versions of equipment they are already familiar with, or equipment designed to work integrated with other materiel they already use. Given the long-standing widespread use of US equipment in Europe, this limits the market opportunities for European suppliers.
5. Interoperability with (non-European) partners: a military may favour a particular equipment because it seeks interoperability with other allies that already operate it. This can create a network effect: the more militaries procure a system, the greater the interoperability incentive for other militaries to follow them. While interoperability is highly desirable and a compelling argument also for European collaboration, it is frequently and successfully used by the US to promote the acquisition of US-made systems, leveraging the fact that the US is the key security partner for most European countries. This is further exacerbated by US suppliers’ and the US government’s refusal to share IP to the extent required for integration of European systems with existing US ones.
6. Absence of European solution: in certain market segments (e.g. MALE UAV, tactical ballistic missiles, and long-range artillery rockets, etc.), there is currently no European solution available on the market, due to European governments’ underinvestment, prioritisation, and industrial policy decisions over the previous decades.
7. Performance/cost: non-European solutions which benefit from larger domestic markets and economies of scale may provide performance similar to European alternatives at a more competitive cost (at least if only the purchase cost is considered).
8. Technical/regulatory constraints: In certain cases, specific technical/regulatory requirements can dictate the choice of equipment to be procured, particularly when combined with tight time constraints.