The bloc's 30 countries will tell their military commanders to draw up a new posture to reflect what Secretary General Jens Stoltenberg called a "new reality for our security" at a summit of Nato defence ministers on Wednesday.
Mr Stoltenberg said the long-term shift in strategy should include stronger missile defence and more battle-ready equipment positioned on Europe's eastern flank to prepare for a Russian attack.
In the shorter term, the US-led alliance already has hundreds of thousands of troops on heightened alert to back up its promise that it will defend "every inch" of its territory against Russia.
But senior figures including Mr Stoltenberg rejected the idea put forward by Poland that an armed Nato peacekeeping mission could enter Ukraine to provide humanitarian aid.
The suggestion by Jaroslaw Kaczynski, Poland’s deputy prime minister and chairman of its ruling party, failed to shift Nato's stance that donating weapons and other equipment to Ukraine was permissible but that direct involvement in the war comes with too high a risk of a wider war with Russia.
Nato has likewise refused Ukraine’s demands to establish a no-fly zone over the country for fear that shooting down Russian planes would provoke a wider European conflict. That same concern helped to sink another proposal of Poland's, to provide a fleet of Soviet-era MiG-29 jets, in a transfer the US described as untenable.
"Of course, we support peace efforts. But we have no plans of deploying Nato troops on the ground in Ukraine," said Mr Stoltenberg of the Polish peacekeeping proposal.