WASHINGTON — President Donald Trump declared a new national security strategy on Monday, stressing the “America first” message of his 2016 campaign and faulting previous U.S. leaders for failing to measure up to it and look out for the nation’s citizens. “Our leaders engaged in nation building abroad while they failed to build up and replenish our nation at home,” he said, pointing to the economy’s strong performance and predicting even better under his policies. His security strategy envisions nations in constant competition, reverses Obama-era warnings on climate change and affirms that the United States will unilaterally defend its sovereignty, even if that means risking the existing agreements with other countries that have dominated the United States’ foreign policy since the Cold War. The strategy from the Republican president could sharply alter U.S.