AMNA NAWAZ: NATO leaders will meet next week and Lithuania, convening nearly a year-and-a-half into Russia's invasion of Ukraine.

Though Ukraine is not a member, the alliance is supporting Kyiv with billions of dollars in weapons and aid.

At issue during the upcoming meeting, revamping collective defense plans and alliance expansion.

White House correspondent Laura Barron-Lopez who will be traveling to the summit, sets the table LAURA BARRON-LOPEZ: In the east and south, Ukraine's counteroffensive to reclaim their land is making slow progress.

Ukrainian soldiers now fire on Russian fighters from positions previously held by Russia.

They seize their trenches, but also their dead.

GOSTRYI, Ukrainian Soldier (through translator): You can see all these flies here and some fresh ground.

Some Russian soldier already rests in here.

It might smell bad as we go further.

LAURA BARRON-LOPEZ: Ukraine's advance comes ahead of a key NATO summit in Lithuania next week, as the near 75-year-old alliance tries to present a united front against the very foe it was designed to defeat, Russia.

JENS STOLTENBERG, NATO Secretary-General: Our summit will send a clear message.

NATO stands united, and Russia's aggression will not pay.

RACHEL RIZZO, Senior Fellow, Atlantic Council: This is probably one of the most consequential NATO summits that we have seen in quite some time.

LAURA BARRON-LOPEZ: Rachel Rizzo is a senior fellow at the Atlantic Council and an expert on European security and transatlantic relations.

RACHEL RIZZO: It's time for the alliance to answer some pretty big questions, not only in terms of their own territorial defense, questions about NATO expansion, but,more specifically, what is Ukraine's future relationship with NATO going to look like?

GEORGE W. BUSH, Former President of the United States: Helping Ukraine move toward NATO membership.

LAURA BARRON-LOPEZ: A critical issue on NATO's agenda, Ukraine's access to the alliance.

In 2008, NATO leaders agreed that Ukraine could eventually join, but there is no clear consensus yet on how and when that will happen.

The Baltic states, formally part of the Soviet Union, want a clear timetable for Ukraine to join NATO.

But the U.S. and Germany have been more cautious, fearing it might escalate the conflict.

RACHEL RIZZO: We don't want cracks and fissures between different NATO allies to start bubbling to the surface.

Not only for Ukraine, but for NATO itself, for the strength of the alliance, it's going to be very important for each and every ally to come together and present a united front.

QUESTION: Mr. President, are you going to make it easier for Ukraine to join NATO?

JOE BIDEN, President of the United States: No.

LAURA BARRON-LOPEZ: For now, the president remains at odds with other NATO members about fast-tracking Ukraine's membership.

But one small step under consideration is removing the Membership Action Plan requirement, known as MAP, for Ukraine.

LT. GEN. DOUGLAS LUTE (RET.

), Former U.S.

Ambassador to NATO: An important part of a MAP, if you will, a Membership Action Plan, is this matter of mature democratic institutions.

LAURA BARRON-LOPEZ: Doug Lute had a 35 year career in the U.S. Army and is a former U.S. ambassador to NATO.

LT. GEN. DOUGLAS LUTE: Ukraine, even before the war, is still an emerging democracy with immature democratic institutions.

If a Membership Action Plan is set aside, there will still be due attention paid to these democratic values.

RACHEL RIZZO: I think there's broad consensus right now that there's no pathway for Ukraine to join the alliance before the war with Russia ends.

The immediate priority is trying to figure out how to continue supporting Ukraine with weapons and economic aid, so that it can eventually be victorious.

LAURA BARRON-LOPEZ: The U.S. is the largest contributor of aid to Ukraine, committing more than $37.6 billion in weapons and other equipment.

But it has been reluctant to send long-range precision missiles known as ATACMS and cluster munitions, until today.

The White House announced it now plans to provide cluster munitions, which are outlawed for use by most countries.

They release bomblets that don't always explode on impact and can leader kill civilians.

LT. GEN. DOUGLAS LUTE: I think we are at a point in the Ukrainian war where there are already many munitions that litter the battlefield.

And the positive impact on the battlefield of these munitions, if provided, outweighs continued concern for unexploded ordnance.

So I think it's an -- overall in our advantage.

LAURA BARRON-LOPEZ: Another big decision expected at next week's summit, the first revamp of NATO's military plan since the Cold War.

Under its new plans, NATO aims to have 300,000 troops ready to deploy to its eastern flank within 30 days to defend against a Russian threat.

And it's charted a specific military strategy for each region in the alliance.

LT. GEN. DOUGLAS LUTE: There will be a specific plan for the northern part of the alliance, so the Arctic, the Nordic countries, the North Atlantic itself.

There will be one for the central region of the alliance and there will be one for the south across the Mediterranean.

And across those specific areas, NATO will for the first time in the last 30 years assign forces.

So they're getting much more specific in response to the obvious Russian challenges.

LAURA BARRON-LOPEZ: But NATO's new plan is expensive, and its ability to implement it depends on members' defense spending.

In 2014, leaders committed to spend 2 percent of their GDP on their military budgets by 2024.

But, so far, only seven of 31 member countries meet that threshold.

The 31st, Finland, is the newest to join since 2020, and it holds the longest border with Russia.

It became a NATO member in April, after Turkish President Recep Tayyip Erdogan lifted objections last year for Finland and Sweden to join the alliance.

JOE BIDEN: I want to particularly thank you for what you did putting together a situation with regard to Finland and Sweden.

LAURA BARRON-LOPEZ: But the process for Sweden has been far slower.

Turkey won't allow Sweden to join unless it extradites Kurdish separatists it considers terrorists.

And President Erdogan says Stockholm's efforts so far, including a new terrorism law, aren't enough.

But President Zelenskyy today warned indecision weakens the alliance.

VOLODYMYR ZELENSKYY, Ukrainian President (through translator): I believe that while the issue of Sweden's admission to NATO has not yet been resolved and the issue of Ukraine's invitation has not been resolved,there is little unity in this.

It is a threat to the strength of the alliance.

And that is why we expect at least some steps toward a positive outcome.

This is very important for the security of the whole world.

RACHEL RIZZO: I think it's looking unlikely that this impasse is going to be worked out before the NATO summit.

But this is going to be a space where everyone is going to be watching very closely.

And let's hope that it doesn't -- if it doesn't happen at Vilnius, it happens soon thereafter.

LAURA BARRON-LOPEZ: For the past 16 months, the world watched as Russian missiles battered Ukraine and the conflict reenergized the alliance, an alliance that Biden has spent the majority of his presidency working to unify.

For the "PBS NewsHour," I'm Laura Barron-Lopez.