The Biden administration has granted approval for the Pentagon to send an additional 3,000 U.S. troops to Europe, joining the tens of thousands of reserves that were deployed there last year in response to Russia’s invasion of Ukraine. The exact timeline for mobilizing the extra manpower and integrating them with the existing reserves stationed to support Ukraine remains unclear.

The authorization from President Joe Biden has been perceived as an indication that the U.S. military’s current training mission in Europe, along with the deployment of several new brigades after the invasion, has stretched active-duty forces thin.

“Reserve mobilizations under this authorization are not to exceed 3,000 total members at any one time, of whom not more than 450 may be members of the Individual Ready Reserve,” Biden said in a letter informing Congress of his order. “These Reserve component forces are to augment the active forces … to enhance the United States’ ability to sustain its heightened level of presence and operations.”

Biden’s order comes after he met this week with Ukrainian President Volodymyr Zelenskyy at the NATO summit in Lithuania. The decision to expand the American military’s presence in Europe aims to reassure NATO allies regarding U.S. efforts to deter Russia from escalating conflicts beyond the war zone in Ukraine.

“Since 2014, U.S. European Command has provided combat-credible forces for rotational deployment to Europe … to demonstrate good commitment to NATO,” said Lt. Gen. Douglas Sims, the director of operations for the Joint Staff. “This reaffirms the unwavering support and commitment to the defense of NATO’s eastern flank in the wake of Russia’s illegal and unprovoked war on Ukraine.”

Last year, the U.S. deployed 20,000 troops to Europe, bringing the total number of American forces on the continent to over 100,000. This includes the presence of 10,000 troops in Poland, which have played a critical role in facilitating the delivery of U.S. aid and supplies into Ukraine.