Strong global leadership can inspire and encourage cooperation over conflict without war even beginning.
Strength and leadership include raw power. Luxembourg will never accomplish anything much on the global stage because it doesn't matter how inspiring and encouraging its leader is, it has no significant power.
Biden is merely the latest president to encounter the harsh realities of the limitations of US power. Every US president since GWB has been staring at the gradual erosion of US predominance in one form or another, most obviously caused by the rise of China. However, numerous other weaknesses have been caused by the USA itself.
1) The Iraq and Afghanistan debacles, which caused significant global dissatisfaction, reduced trust in and the reputation of the USA. That they were in large part failures showed the world the limitations of the USA, heavily increased anti-intervention sentiment in the USA on left and right, which led to further half-hearted operations that increased the sense that the USA was weak.
2) Donald Trump inflicted substantial damage to US reputation with grotesque and ham-fisted foreign policy, particularly amongst US allies. Bluntly, a country is always going to struggle to lead when it's just alienated those it seeks to lead.
3) Internal dissent: political partisanship leaves the US president weak. The president has very little room to manoeuver due to opposition and is routinely undermined in his own country - a leader weak at home will be weak abroad, too.
In particular regarding 2&3, Putin cannot fail to have noticed that many Republicans would de facto rather take the side of Russia than their own president, just so long as the president is a Democrat. Such as Trump releasing press releases on how he wants to have Putin's babies, and barely any Republicans can be found to criticise him except the ones censured by their own party.
In a sense, therefore, Biden really is weak. But he's weak in large part because of institutional factors within his own country, and any US president would almost certainly be in the same position.