The short answer
No one can honestly list the countries in a future world war, and we won't guess. What we can show is who has promised to help whom. The UK is one of 32 NATO members. Under Article 5, an armed attack on one member in Europe or North America counts as an attack on all, and each member takes the action it thinks necessary, which may or may not mean armed force. Article 5 has been used once, after the attacks on the US on 11 September 2001. The UK also leads a 10-nation Joint Expeditionary Force in northern Europe.
The honest answer: no one can list them
Searches for "WW3 countries list" have jumped. It makes sense to want a clear answer. But nobody can tell you which countries would fight in a future war. It would depend on what started it, where, and what leaders decided at the time.
What we can do is show what is written down: which countries have promised to defend each other, and what those promises really say. We don't speculate about enemies or targets.
Who the UK is bound to: the 32 NATO members
NATO lists 32 member countries. The UK was one of the 12 that started it in 1949.
| Joined | Countries |
|---|---|
| 1949 (founders) | Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, United Kingdom, United States |
| 1952 to 1982 | Greece, Türkiye (1952), Germany (1955), Spain (1982) |
| 1999 to 2009 | Czechia, Hungary, Poland (1999); Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia (2004); Albania, Croatia (2009) |
| 2017 to 2024 | Montenegro (2017), North Macedonia (2020), Finland (2023), Sweden (2024) |
Some European countries are not on NATO's list, for example Ireland, Austria and Switzerland.
What Article 5 actually says
Article 5 of the North Atlantic Treaty is the core promise. In short:
- An armed attack against one or more members in Europe or North America "shall be considered an attack against them all".
- Each member will help "by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force".
- The aim is to "restore and maintain the security of the North Atlantic area".
- Any attack and response must be reported to the UN Security Council.
Article 6 sets out where this applies: members' territory in Europe and North America, and their forces, ships and aircraft in certain areas such as the North Atlantic and the Mediterranean.
What Article 5 does and doesn't do
| It does | It doesn't |
|---|---|
| Treat an armed attack on one member as an attack on all | Automatically send every country's army |
| Commit each member to help | Say what that help must be. Each member decides |
| Cover attacks in the treaty area | Cover every place in the world. The UK's risk register notes some UK Overseas Territories may fall below the Article 5 threshold |
| Need allies to agree it applies | Switch on by itself. The risk register says it is "context-dependent and relies on NATO political consensus" |
There is also Article 4. Any member that feels threatened can ask the others to meet and talk. NATO says this isn't a required step before Article 5.
The one time it was used
NATO has invoked Article 5 once, after the terrorist attacks on the United States on 11 September 2001. On 12 September the North Atlantic Council agreed the attacks would count if they were directed from abroad, and it confirmed this on 2 October.
The help that followed was mostly support: sharing intelligence, opening airspace, ports and airfields, and NATO surveillance planes patrolling over the US. A naval operation in the Mediterranean ran from 2001 until 2016. This shows that Article 5 help can take many forms.
The UK-led Joint Expeditionary Force
Separate from NATO, the UK leads the Joint Expeditionary Force (JEF). A Ministry of Defence release from 2 October 2026 lists 10 nations: the UK, Denmark, Estonia, Finland, Iceland, Latvia, Lithuania, the Netherlands, Norway and Sweden. Its stated aim is "to secure the Baltic Sea, North Atlantic, Arctic and High North for NATO". The same release says ministers would discuss Canada's application to join.
The release doesn't describe a mutual defence promise like Article 5, so we don't claim one.
What the UK government plans for
The National Risk Register 2026 includes a scenario where an adversary invades a NATO ally or attacks UK forces abroad, and all allies agree to invoke Article 5. It describes:
- disruption to UK and European economies
- possible severe disruption to supply chains, fuel and energy
- likely more "hybrid" attacks on the UK, such as cyber attacks
A second scenario covers a conventional missile and cyber attack on the UK mainland. Its listed effects include disruption to essential services such as food, water, energy, transport and communications.
These are planning scenarios. They are not predictions.
What this means for your home
Who signs which treaty is out of your hands. What you can control is how ready your home is for disruption of any kind. The government's Be Ready advice is the same whatever the cause:
- 3 days of water and food. 3 litres of drinking water per person per day.
- A battery or wind-up FM radio and spare batteries.
- A torch and a charged power bank.
- Cash, copies of documents and a list of medicines.
Try our 2-minute Ready Check to see where you stand. For what officials say about the risk itself, read is WW3 likely? and is the UK at war right now?. To see how other countries talk to their citizens, read what Sweden, Finland and Ukraine tell people.
What to buy
About these links: plain links to ordinary retailer pages. We may earn from some links in future; we don't now. We haven’t tested these and we don’t rank them: they are examples that were in stock when we checked. Prices change. Check what you already own first. Full kit list
Battery or wind-up FM radio
Look for: FM (Be Ready asks for FM; DAB as well is fine). Both a crank and ordinary batteries, so you have two ways to power it.
Spare batteries
Look for: The sizes your torch, radio and alarms take. Check the date on the pack.
Torch or head torch, plus spare batteries
Look for: Runs on the same AA or AAA batteries as your other kit, so one spare pack covers everything. A head torch leaves both hands free.
- Alpkit Ionic 350 Lumen Head Torch · Alpkit · £15.99 (price on 1 November 2026) Shop
- Lifesystems Intensity 155 LED Head Torch · Lifesystems (UK brand store) · £19.99 (price on 1 November 2026) Shop
- Alpkit Kula 550 Lumen Head Torch · Alpkit · £19.99 (price on 1 November 2026) Shop
- Lifesystems Intensity 480 LED Hand Torch · Lifesystems (UK brand store) · £29.99 (price on 1 November 2026) Shop
- Dynamo Powered Wind-Up Pocket LED Torch · EVAQ8 · £3.60 (price on 1 November 2026) Shop
Power bank
Look for: From a seller you know. Check the capacity (mAh) against your phone, and that you have the right cable for each phone. Keep it charged.
Frequently asked questions
What countries will be in World War 3?
No one can know. Wars depend on events, leaders and choices that haven't happened. What is public is which countries have promised to defend each other. The UK is in NATO, a group of 32 countries in Europe and North America that have agreed to treat an armed attack on one as an attack on all.
What does NATO Article 5 say?
It says an armed attack against one or more members in Europe or North America shall be considered an attack against them all. Each member will help by taking the action it deems necessary, including the use of armed force, to restore the security of the North Atlantic area. Each country decides what its own help looks like.
Has Article 5 ever been used?
Once. NATO invoked it after the 11 September 2001 attacks on the United States. Allies agreed support measures such as sharing intelligence, opening airspace and ports, and sending NATO surveillance aircraft to patrol over the US.
Does Article 5 mean every NATO country must send troops?
No. The text says each member takes 'such action as it deems necessary'. That can include armed force, but it doesn't have to. The UK's National Risk Register also says triggering Article 5 depends on the context and on political agreement among allies.
Would the UK definitely be involved in a big war?
No one can say that. The UK has treaty promises to its NATO allies, and the government plans for scenarios where Article 5 is invoked. Those scenarios are planning tools, not predictions.
Sources
We opened each page on the date shown. Official advice changes: if a source says something different now, follow the source.
- NATO: The North Atlantic Treaty (Washington, 4 April 1949), Articles 4, 5 and 6 (read 1 November 2026)
- NATO: Collective defence and Article 5 (last updated 12 November 2025) (read 1 November 2026)
- NATO: NATO member countries (last updated 11 March 2024) (read 1 November 2026)
- Cabinet Office: National Risk Register 2026 (PDF), 'Attack against a NATO ally or UK deployed forces, which meets the Article 5 threshold' (page 221) and 'Conventional attack on the UK mainland or Overseas Territories' (page 222) (read 1 November 2026)
- Ministry of Defence (GOV.UK): UK leads major Joint Expeditionary Force exercise as defence ministers meet in Iceland (2 October 2026) (read 1 November 2026)
- GOV.UK Be Ready: be ready for emergencies (checklist) (read 1 November 2026)
Related guides
- Worried about war? How UK households can prepare — calmly
- Why is the government asking us to prepare?
- The Be Ready checklist, explained for your home
Free tools
- How ready is your home? The 2-minute Ready Check
- Be Ready kit: every item on the official list as a tick list
We're an independent UK site, not the government. We check guides against official sources and link to them. In an emergency call 999.
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