It will not be necessary for new members to buy a lot of Western equipment, but they must be able to communicate. Nato has committed itself to not extending its "infrastructure" into the territory of new members. But, realistically, some new infrastructure, if only secure communications, will be necessary. A fundamental question is whether applicants have enough people who can speak English and French - and whether they have a professional corps of non-commissioned officers.
Can they pay?
In spring, all Nato countries will investigate credentials of potential new members. That will include analysis of the costs of enlargement. In addition to bearing the cost of upgrading their armed forces, new members will have to pay a "subvention" to Nato budgets. The UK currently pays about 10 per cent of the budget and the US 25 per cent ($147m). Contributions will fall if new members join. But US sources insist new members will "not be required to fight World War III" - so contributions can be kept modest.
Will they agree?
Nato enlargement has to be ratified by legislatures of all 16 members, including two-thirds of the US Senate. Powerful interests in Turkey have threatened to re-fuse to ratify if Turkey's ambitions to join the EU are frustrated. A majority of one against in any national parliament could veto. Each member judge on its own criteria, and the 16 must be unanimous.The US, which provides a quarter of Nato funds, will have influence.
Can they all join?
Realistically, Nato will keep the invitation list as short as possible: the Czech republic, Hungary and Poland are likely. Slovenia and Romania are possible. The Baltic states (Estonia, Lavia and Lithuania) would like to join. But Russia would regard this as unacceptable. Russia might be prepared to live with the accession of the most westerly countries of the former Warsaw Pact. But Nato is adamant the door must remain open.
Will Russia react?
Nato insists enlargement will happen, whatever Russia says. In fact, Russia cares very much. A Nato-Russia "charter" is seen as a vital precondition to expansion to help assuage Russian concerns that enlargement is hostile. Madeleine Albright's proposal for a joint Nato-Russian brigade is another indicator that Russia must be brought in from the cold. Russia will want other concessions, too: progress on the Treaty on Conventional Forces in Europe and reassurances on nuclear weapons.
Will Nato weaken?
Getting 16 nations to agree takes time - Nato found this out in Bosnia. Nineteen or more could be more cumbersome. Nato officials insist there will be no "second-class" members and the Nato security guarantee - an attack on one is an attack on all - applies to all. Nato leaders insist there is no point in expanding if it makes it weaker, though many would argue enlargement will do so. The US stresses Nato was always diverse, and that if it is going to stand still at 16, it might as well not exist.