NATO leaders are gathering in the Lithuanian capital, Vilnius for their annual summit. They will meet starting Tuesday and, despite their differences, are trying to present a united front.

The leaders will discuss adding new members to their ranks. They are set to drop some requirements for Ukraine's bid but are divided over other details.

"The most urgent task now is to ensure that Ukraine prevails as a sovereign and independent nation in Europe," NATO Secretary General Jens Stoltenberg said on Monday. "Unless Ukraine prevails, then there is no membership issue to be discussed at all."

Officials in the Kremlin say Ukraine's membership would represent a threat to Russia. Spokesperson Dmitry Peskov said such a development would require a "firm response."

NATO leaders are already wrestling with how to ensure Ukrainians can protect themselves. US President Joe Biden has decided to make up for a lack of artillery shells and send cluster bombs. But his partners in the United Kingdom, Canada and Spain have warned of the dangers to civilians.

The leaders will reach back to the Cold War. They are set to discuss plans for their collective defense in the event of a Russian attack on one of their members.