NATO leaders say bolstering the alliance's forces in Eastern Europe as a bulwark against Russia is a key priority for the Madrid summit.
British Prime Minister Boris Johnson said as he arrived for talks Wednesday that NATO needed to learn the lessons of the last few months and the need for NATO to revise its posture on its eastern flank.
Polish President Andrzej Duda said NATO's commitment to greatly increase its rapid reaction force for members nearest to Russia will make Europe safer.
He said: Russia is a threat for Europe and not only for Europe, but for all of NATO.
NATO has said it will boost its rapid response force from 40,000 to 300,000 troops for its eastern flank, which includes Poland's borders with Ukraine and Belarus, a Russian ally.
The summit's opening work session on Wednesday will focus on how NATO can give more support to Ukraine.