The biggest spurt of growth that NATO experienced since its very founding in 1949 occurred during 2004, after years of intense dialogue.
This growth, while not exclusively the doing of the United States during the 90's and 00's, is one of the greatest geopolitical victories that the United States has seen in the past few decades (even if the benefits have not yet been realised).
In 2004, NATO absorbed all three Baltic states (Estonia, Latvia, and Lithuania), as well as Slovakia, Romania, Bulgaria, and Slovenia. As you can see in the image to the right, NATO thrust itself towards Russia, essentially leaning right up to St. Petersburg (imagine if the Warsaw Pact had a stronghold a hundred miles from Philadelphia or Chicago).
This expansion essentially means a new, strengthening front line in what is an increasingly important and difficult effort to keep the Russians locked up. In time, these countries will likely become allies as strong as Poland or the Czech Republic, allowing the United States to think about its strategic borders on Moldova and Belarus, rather than further back. NATO also gives these countries opportunities to experience economic growth with the European and American nations with whom they have grown cozy, eliminating trade barriers and creating new markets and providers.
These strategic partners will grow as the United States' relationship with Western Europe grows cooler over time. As Western Europe more and more sees little need to be taken care of by the United States, it will continually assert itself (especially through the EU) towards a greater independence by pushing back against the US and emphasizing differences of opinion. With Eastern Europe set up as key allies in NATO, the United States' strategic focus will drift eastwards, focusing on Warsaw instead of Paris (the UK is likely to grow closer to the US as well, as it moves to assert its independence with respect to the European continent).
But these Eastern European countries, including Croatia and Albania (which joined in 2009) will be willing and enthusiastic front-line soldiers against Russia as it tries to assert itself on the European continent and undermine US machinations there. They will enable the US to hold its strategic position in Europe via its favorite and most effective strategy--to find smaller allies and spend money on their economy and military.
While folding Ukraine and Georgia into NATO would have been a nearly-fatal blow to Russian regional ambitions, the incorporation of most of Eastern Europe into NATO before Russia was strong enough to oppose it will prove to pay off major dividends into the 2020s, as Russia's military might and aggression are likely to peak. Poland and the region around it will become the key allies of the United States well into the 21st century.